Well firstly well done on buying such a spectacular plant! I have six, now seven as I bought an interesting one yesterday. I've been growing them for a number of years now, at least eleven and am slowly discovering what they need and don't need, though I haven't managed to kill any yet.
All of mine are grafted onto japonica stock. The first one was a tall plant, three meters with a very thick trunk, having about six large Camellia azalea branches grafted onto the top. This has been a real treasure and hasn't stopped flowering since I acquired it. Unfortunately the stock (Camellia japonica) didn't seal over and some dry rot set in and over time some of the grafts died back. It's still alive, three large branches of it and still throwing absolutely stunning flowers the size of Magnolia blooms (4.5 inches tall x three inches wide), same upright habit, fluted in the most ravishingly beautiful electric scarlet. Im not sure how long it will continue as it doesn't have many leaves left but the few it has are very large leaves. There was initially some leaf drop which also worried me, not many but quite regularly until I stopped noticing. I planted in a large terracotta pot using pure yellow/orange local forest subsoil which is acidic and slightly sticky. When dry it's rock hard when moist soft and crumbly but always quite open, on top there is a mulch of fine bark. The soil it came growing in was dense grey clay which is still in the pot. Rainfall in Hong Kong is particularly high so I don't water much in summer but if we have a week of sun in summer (very unusual) I will water thoroughly. Winter I hardly water at all only during very warm weather as they need a dryish season. However we have to bear in mind that the bottom half must be treated like Camellia Japonica and the top half like Camellia azalea which is quite tricky in Hong Kong.
Hong Kong's climate is more or less exactly the same as where they are found growing wild, just across the boarder in Guangdong. So thats all well. Unfortunately Camellia Japonica's needs are quite different here as they don't come from Southern China. They do grow here reasonably well provided you give them some basic assistance like shade from the hottest sun, and it can be very hot especially around June July August..... So to this end I keep the trunks and roots shaded with the grafts or top part more or less in just under full day sun. I achieve this by having them "behind" sun speaking at least, a ficus hedge, the garden path between for easy access. That way the suns angle only strikes the top of the plants not the standard or the roots.
Because of the grafting problems of the first one I bought the next two I bought were clean top grafts on a much more slender trunk stock, one to one no exposed wood. Same flower and leaf form as the first one. Basically a single large branch grafted cleanly onto japonica stock. These are doing well in the same soil however I did mixed some peat into the soil this time, not much 2/8. So it's basically pure soil very little organic matter. You can find peat at Brighten stores. Coconut husk compost Im a little worried about here but it is at least slow to break down. I've tried it with a few things but smelly root rot ends up a bit of an issue, apparently it needs NOT to be sterilised in its manufacturing process because that removes any antiseptic qualities it had before and Im not sure what we get here.
Camellia azaleas have an incredibly diverse phenotype for such a small population of wild plants, they also have a very narrow genetic diversity however. So I keep seeing flowers and leaves which are very different. The actual growth form can vary widely as they are branches that are taken from wild plants and grafted onto japonica. Im not entirely sure how sustainable this can be either. Leaf shape and size can be dramatically different, as can form size and colour of the flower. I tend to prefer the deep deep bright scarlet on the large magnolia type flower with the large slightly pointed glaucous grey/green leaf. The flower doesn't open so widely, much like a tulip and has more texture and saturated colour. Some of the smaller leaved ones also have flowers of a "good" colour but the flowers are much much smaller and open wider like a cart wheel. They range in colour from almost shell pink to faded red to deep scarlet. The deepest coloured ones always sell first and you will often see the paler pinky reds sitting there for years at a time. You need to act quickly when they come in. I haven't seen that many of the large flowered deep scarlet one for a number of years since I got my first four. Dont expect anyone to know what you are talking about if you try and order one to your specifications....it doesn't work they will tell you they are all the same colour even though whats in front of you patently makes that untrue.
I have experimented with shade and sun, acidity and fertiliser and it doesn't much change the colour they come with or the growth of the plant, though spraying with a high potassium foliar feed seems to brighten things up somewhat as does planting in the ground. The last two I bought were one a pleasant pinkish red, large cup shaped flower with broad short rounded petals and oddly enough leaves, looking very different, and a tiny flowered deep deep scarlet with many very small very pointed leaves.
Apparently the soil they grow in is high in potassium, is subject to seasonal flooding, literally they stand in water but it's moving water, its along a river but is otherwise bone dry not moisture retentive at all....not at all the same as japonica. So as its japonica stock you have to treat the roots as japonica and more or less hope for the best for the top.
I haven't had any problems with leaves turning yellow, or any of the camellia problems, bud scorch, blight, black spots etc they seem really quite tough. I wouldn't worry about leaf drop its quite normal, the leaves don't seem to go through a yellowing stage before they drop, mostly these are the oldest leaves and run the course of their life. When they drop they are clean and dark dark green or in the case of some grey glaucous green, just old, occasionally slightly desiccated brownish in part. As long as the roots are healthy everything should be good. If its a drastic leaf drop, a rain of healthy leaves then your roots could be rotten. Before you buy, well before I buy the last thing I do is gently lift the whole thing out the pot and inspect the roots briefly. They don't like you doing it but its not difficult to do as the soil is such hard pan clay , generally all it takes is holding the pot down with one hand flat over one side of the rim and lifting the trunk up by the base with the other. Before they can object you know exactly what's down there. If its wet and soggy and you can't see any roots just big gaps and slush maybe not. Nice living roots and a sturdy root ball is a lovely thing to see in Camellia azalea.
I feed a balanced slow release, often and little, with our high rainfall it's the safest thing to do. Think it's a 13-13-13 osmocote type thing. It comes from Japan and has trace elements but not made specifically for Camellias. I also spray feed with a 10-0-25 + 2 and trace elements, usually when Im doing the orchids, so it's very dilute. This I get from Thailand. I have also been known to pour a little yellow sulphur (flowers of sulphur) dissolved in water over the roots. They seem to thrive. Though being so seemingly tough Im not sure if I couldn't be doing better. They aren't exactly fast growers, so don't expect a sudden new flush of leaves as they're too busy flowering most of the time, but you do need to stimulate growth or they will flower to death. I do this mostly in Spring with a higher nitrogen, also dilute, little and quite often until they start producing leaves.
I also use a ground cover over the soil to keep it cooler, various things that don't have deep roots. The mulch I use is a very very fine pine bark mulch, almost like tea leaves, it needs replacing once a year as it breaks down quite rapidly. Recently I haven't been able to get such fine bark but the slightly bigger one, about one inch seems to work.
How long they will live as grafted trees is anyones guess. However typhoons and extremely rough handling by dealers who will often pick them up by one side branch above the graft doesn't seem to affect them too badly. Though if you look carefully you can often see wear and tear where they've been picked up as the bark can be worn away completely smooth. So selecting is quite lengthy process and usually you have to wait and wait for the right colour, form, graft and general health. Sometimes there are cracks in the bark of the stock too. Don't let the sellers handle them and after you made your decision pick them up by the pot even if you need a crane!
Good luck hope it thrives for you. ps terracotta allows the pot to dry out fairly quickly in our hot rainy weather, it breaths, perspires and water is drawn through the porous walls out thus also keeping things cool , so you won't get much by way of root problems even with loads of water. Eventually a nice layer of moss grows on them too, so the whole thing becomes a living happening. Just don't bury the neck and don't over feed, keep the roots shaded and cool. I think local soil is the way to go as you don't want to be repotting and fiddling with easily degradable potting mixes high in organic material, that very quickly leads to trouble and rot. They very quickly look completely established as though growing all on their own in that spot for years. Dont move them about either, find the natural way they were growing towards the sun and stick to it.
Lastly I've found that dead heading helps with the emergence of a new flush of leaves enormously. The petals will drop off whole in one piece leaving behind the sepals ovule etc this you need to dead head by snapping between finger nails at the flower stem. They look like empty buds, if you press them gently you will see they are whats left of the spent flower, they will also be whitish green and not so pointy. They very rarely set seed and often the seed is infertile. So its best to remove that which stimulates a set of leaves behind. Otherwise it stays on and becomes dead and woody and the new leaves only start growing when it falls off. So just snap of the spent buds at their stems. It's a little difficult because the buds are produced in tight clusters and you don't want to remove new buds but once you get the knack it's fairly easy, the new buds don't fall off easily. I generally don't do it religiously every time a flower falls but every couple of weeks I go over them all when I notice a light carpet of flowers on the floor.
Its a magnificent Camellia and once you have one you like you will adore, they truly are ever flowering in our climate, those clusters just keep pushing out buds and while some branch tips are growing leaves, others are still flowering........I haven't noticed a seasonal flush as it were of anything except it slows down in our cool dry season and takes off again when it warms up and the rain starts a bit again in May.
Hope that helps a little. So little is written about them it is difficult knowing exactly how to proceed with these.....sure your own experience even after a year will come in very handy to anyone else starting off too.