Great choice of boat, can't really go terribly wrong:) but the SJ21 is pretty old, so you can find some issues.
delamination of the core is a bitch... Lots of articles on the net on how to check for it so do a little research on that. Weak areas i've noticed are the bunks in the cockpit where people step down from the cabin area. The core below the mast step, the area around the Jib tracks, and pretty much anywhere a fitting has been installed.
Look over all the fittings, boom end caps, mast, etc, etc... stainless can rust, and after 30+ years some of these parts can be weak if they were neglected. easy enough to replace, but something to think about when negotiating price.
I can't think of any absolutely "killers" that can't be repaired with a bit of time and money. Something that is totally out of class spec might dissuade me. Someone local found an SJ21 that had it's keel broken at some point, so the owner had an "envelope" of stainless steel welded up, then had all the lead from the stock keel poured into it. Somehow he rigged it up, and off he went. Obviously it's out of class spec:)
Bent mast's are no fun either. I'm not sure what the going rate is right now, but you can probably check at sjsailboats.com. I think Stephen has mast prices listed up there.
A few thoughts on the Fleet 1 site...
http://www.sj21fleet1.org/tips-and-tric ... ce--spares
http://www.sj21fleet1.org/tips-and-tricks/keels
http://www.sj21fleet1.org/tips-and-tricks/deck-recore
Anyone else got any other thoughts?
Later
C