If you stick with R12 and the freon isn't leaking, replacing a bad compressor in itself is no big deal depending on why it died in the first place. As long as there's no metal particles in the AC lines, no common either BTW, there's simply no reason why you can just replace the compressor only.
Not only do I repair Volvos daily for a living, I also own an older 745 and a few years ago I replaced my original AC compressor because its seals were leaking freon. I installed a used compressor and it's been fine ever since.
I don't ever advise converting an R12 system to 134a unless you're usually leaking freon and a DIY who can just add his own 134a whenever needed (legal in some states). Sure, R12 costs more per lb but the stuff's still very available and if it's not leaking, is cheaper to use than converting in most cases.