>>28476673 (OP)Old R12 systems are the shit. Can be filled with propane (r600) instead if you don't want to pay hundreds for r12.
Converted r12-->r134a systems are typically hot (and) garbage, avoid if possible.
Older stock r134a systems (mid 1990's) are fine.
Of course, all of these systems are 30+ years old and will most certainly be on their way out even if you paid top dollar; always budget for an AC overhaul