For reasons that would take too long to go into (tl;dr version - a combination of mass gradual soil movement needing soil engineering, bad development, old paving/topslabs, + ground levelling to address long term ground issues) the entire top 20-80cm (8-30 inches) of old paving, sand and soil under it is being stripped from a garden (approx size 6 x 14m), and will be replaced with new topsoil when structural and soil movement issues are remedied. Because of the nature of the issues, its not practical to keep the old soil and reuse it (tl;dr #2 - no storage space for such a volume while works going on, economics prevent doing it bit at a time). The ground below is a deep clay soil. The old ground level was sloped, so the depth removed will vary from place to place. The location is close to London, England.
The work is underway, so whether a better way might have existed is pointless now, this is how it is. So the end result will be a new topsoil layer. The work should prevent soil movement in future but water may still tend to flow in the soil in one direction, as adjoining land will be higher in one direction than the other.
My question is about the choice of new topsoil. We get to choose what to put back when the work is done. I don't know if there are different kinds of topsoil, or some that have organic matter mixed in for worms, or whatever else is needed to allow a healthy soil ecosystem to be established. Perhaps some topsoils are "new" and sterile and others are "recycled" and more varied, but I don't know if so. I have the possibility to choose a specific kind of soil, or to premix it with nutrients. While the soil will mostly be laid with grass seed, we would like a fair bit of fruit plants and maybe a fruit tree or two in it, but even if we didn't, we'd like to help the soil to be established with a helping hand toward a thriving subsoil, that is also sustainable (ie reduce nutrient leaching and help nutrients to replenish), and a good soil, whatever that means.
What should we do / what are the options?
@stormy - see reply to @Bamboo below, the original question contains links to photos before the work. The garden was originally on a slope and had been cut away. While the houses are in good condition for over 100 years, all the gardens on that slope have subsidence/soil movement, and show cracks/bulges/failures in retaining walls and some have substantial retaining concrete. Because of other circumstances time was very pressured and foundations needed redoing in 2 places anyway, so the ground was dug and a new engineer-designed retaining wall will go in. Before the heavy equipment all goes, and while I still have access for large items > 1.5m wide, it's a good time to get into the garden, whatever will be the new top layer of soil.
I haven't heard of seed spraying services, can you point me to any (I couldn't find links). The main reason for seed vs sod is cost, after so much unexpected work the economics override most other things. If seed can be done, then the cost saving will be a big help. Water in this area flows towards the houses and into water drains rather than into next door gardens, but because gardens are at different levels there may be flow of subsurface water from higher to lower gardens as well. I can put in trenches easily.
There isn't a chance to design the beds at the moment, there's too much else to do! So I want to get the topsoil "about right" and the hardest work done. I might need to leave it idle and covered (or even in its large bags) until the Spring season while we fix the rest of the house structural issues, then come back to it in a few months and remove any weeds, plant seed, decide beds/ditches and anything else. But it will be easier to get the soil in now, while I have access for large/heavy items.
It turns out that a 2.5 ton digger makes a superlative soil breaker if the bucket is jiggled and the soil allowed to scatter out of it. The soil looks a lot better than I expected, now that the broken glass, rocks and weeds/debris are mostly dealt with. A lot will have to be removed for structural reasons and levelling, but it should make a good soil. I may need to mix something or other in, or put a few inches of topsoil over it so the original soil is not near the surface (I found quite a lot of broken glass throughout, and no way to remove it all). Essentially the digger and crude removal is getting quite a nice soft crumbly mix. Probably needs fixing somehow as its clay and will quickly clump again and be non-draining, but I'm fairly confident now that I could mix in whatever's needed to help with that, in a matter of a few days. I'm down to bare soil throughout, and whats left to do is trenches for structural concrete work, levelling and putting in structurally viable subground retaining walls against soil movement (originally sloped/cutaways at boundaries), and returning the garden back to being a garden not a building site.
So my previous assumption that it's not practical to keep the old soil has been changed. If I sift+keep the needed amount of the existing soil instead of getting new, and I want to modify it to be better in terms of nutrients, drainage, or other qualities, I can probably do it instead of buying new, and mix in whatever is needed instead - but what is it that I should be doing?