You look for rust rust rust. Inner fenders, rockers, subframe assemblies, firewall. I mention these cause they are the toughest to fix. Bring a strong light , safely jack up the car (not a little tire changing jack) and spend some time on your back looking at everything. rust free may just mean you can't see the rust from the outside...pull up floor carpets, bottom edge of rear hatch...bottom edge of al doors. Evrything else is fixable....but I would rather spend $5-10K on a a true rust free wagon, then start for free on a rusty one. Oh yeah after rust a decent interior is the next hardest thing ($$) to sort. Mechanicals are easy although I guess rear difs and trannys are getting slightly harder to find. $1200 is a decent price if the car is really rust free. Try to bring someone with experience vis a vis old cars and rust with you. Did I mention rust?
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Patrick, '68 220, '92 Eurovan , '53 PD4104 (highway coach conversion).