I bought a replacement second hand case and a brand new keyboard so that it looks almost as good as new, apart from the warped case at the bottom left, which I've noticed has happened to my brothers as well, must be a heat thing.
I have a few 1200's, and they all have that same gap where the cases join in the front middle. For example (from
www.commodore-amiga-retro.com):
You can just detect the affect even from this 1995 advert for a brand new Amiga:

And this one is a photo of a brand new NOS Petro unit, with the gap clearly visible:

So all in all I think they were like it from new. With age you do get a sagging affect where the whole unit saggs under its own weight (even worse if it's been stored in its polys)...and to fix this I fit an extra rubber foot in the middle of the Amiga towards the front, but set back from the front middle screw. This I don't believe has anything to do with the gap though, but I may be wrong. Personally I think the screw holes are cut slightly out of place so it "squeezes" that section. Anyway... definitely not heat related.
When storing Amiga's in their polys, I always store them upside-down; this prevents the unit from sagging, and helps revert it if already present.
Congratulations on building your new Amiga. I hope it gives you many years of fun. :cool: