I'm by no means an expert, but having played with head replacement lately, I'll try some advice.
The main body of the trimmer will likely have options for larger engine sizes, which might limit the size of brush it can be used to cut. This will principally be a factor of the installed head, however. Also, different power source/fuel mixtures can be easier to work with. Personally, I'd like a 4-stroke engine so I can use the same gas tank as my lawn mower, but I've currently got a 2-stroke unit. Different 2-stroke engines require different fuel-oil ratios. I don't know that any engine functions significantly better, but things to take note of while looking.
There are many different types of head you can get for a trimmer. A string head works decently on small brush and heavy grasses, while a bladed head can cut thicker stalks. Heavier bladed heads can cut thicker brush. If you're planning to use this device for multiple purposes, look for something which can easily swap heads. If you're adding a bladed head, look for something in which blades can easily be replaced.
I've seen several bladed heads which say not to use them vertically. I'm not sure how much overlap their is between string trimmers and lawn edgers, but if you'd like to do both I'd definitely research carefully before purchasing.