Forget about converting a C70 Coupe to AWD. Not only would I say that it's almost impossible (or at least impossible w/o spending ALOT of money, providing you could find anyone who'ld consider doing it (unlikely), if you read a few AWD (XC) threads around here you'll soon discover that Volvos AWD system (prior to the new one that JUST came out in the AWD S60) is not exactly a trouble free or desireable system. Volvos XC AWD system has been known to have several "issues", none very good IMHO.
Few people also really NEED AWD anyway, I surely don't so it's not a good option for many, I'd say.

Now.....the C70, your only main possible problem with owning one will be the doors window regulators as it's a terrible design and very costly when one breaks (and if you own it long enough, at least one WILL break). Thin metal wires rolling through plastic wheels just can't last too long and that's what they used for regulators, great heh? The part is costly and labor to do it right (a royal pain to adjust w/o leaks) is also costly if not warranty.