The only items I would worry about on the older ones would be 1) fret wear, 2) wood cracks in the neck pocket if a bolt-on model, and 3) unwanted modifications. Some sellers understate fret wear, or even try to hide it by raising the action. Wood cracks in a bolt-on pocket can be a fairly expensive repair. I read somewhere that the usual "fix" is basically setting the bolt-on neck. Modifications can be good or bad. I'm not sure I'd take a chance on a modified PRS without playing it first unless the seller has a return policy (or at least includes the original parts).
All that said, my 1990 EG has serious mojo that my 2008 just doesn't have yet, and it will be worth every penny when I end up having to get her refretted. If her neck pocket ever cracked, I'd pony up for whatever the fix would cost me, too.