I'd start with the Kombi system. You could save a few bucks with the FS90, but with cheaper bearings an a wimpier shaft, it's not rated for attachments.
An FS90 with an Edgit might be OK for now. But a string trimmed walk isn't the same as one trimmed with a blade. So get the Kombi with a string trimmer for now. Trim as best you can without an Edgit. Put the money you'd have spent on an Edgit toward a real edger head for your Kombi system.
Then who knows, in the future you might want a little pruner chainsaw for your Kombi system. Or a hedge trimmer attachment. I have one of each, and they are quality professional equipment that will last.
Just remember one rule. With Stihl equipment use synthetic 2-cycle oil. And don't get generous. Mix it like it says on the bottle, not what's in the Stihl manual.
I don't mean to hi jack the thread but since you brought it up, I have two fs 90's one I use with a hedge trimmer attachment, is the 90 to small for this attachment?