They made about 8,000 of them. Brett has those numbers on this site that you could look up. I know they made 880 with the 403. For a comparison, I have a pretty much solid body car that does need the front floor pans replaced (it appears that the kick panels leaked), new interior, and paint....plus lots of mechanical work. But there is no other rust in the body. The engine and transmission are sound. I paid $2,800 for my Y88 403 last October. It was pretty much complete but there are a few things I had to scrounge up.
Expect to put a lot of money into it. That way you won't be disappointed if you do, and you'll be happy if things turn out better than you thought. But they are beautiful cars when they're freshened up.
Also, make sure it's a Y88 and not simply a gold T/A. One way to check is the T-Tops have a gold tint in the glass and they actually say gold on them. I know these can be added, but there are other clues to look for sans a build sheet. If you're really in doubt, pay for the PHS document on the car and you'll know for sure.
If it's too rusty....pass on it. Find one that isn't.