Some things to consider:

Where you are at - this affects both heating and cooling. If you don't have access to junk cars to get cheap parts and do a little experimenting, I would assume that the heater and A/C could not be restored to operating condition cheaply. You might get lucky and a relay or fuse might fix the problem, or not so lucky and require some major repair work.

Do you freeway drive - if so you will want to fix the OD. I drove for a week with no OD (20 miles one way interstate driving) - it drove me crazy and I spent the extra time to dig way deep into the OD problem. It turned out to be a bad switch, bad wiring from the switch to the relay, a bad fuse, and bad fourth gear switch. I only spent $50, fixing it, but I spent a lot of time digging the bugs out. I consider myself very fortunate as nothing mechanical was wrong with OD system and the $60 relay was good.

Suspension condition - my wagon has bad bushings in the back and is driving me crazy (it "thunks" everytime I let off of the clutch). Reputable local Volvo specialty shop quoted me $500 to fix (rear only). I will buy bushing kit for $90.00 w/ urethane bushings and do work myself - I expect to blow most of a weekend on this one.

I enjoy my brick and feel like I got a lot of car for the money. It is basically good, but had enough "issues" that I could learn some valuable info about Volvo's. If you can get your car for under $500, it would be worthwhile if you take the above things into consideration and expect to spend some money and a lot of time working on it. I have found that neglect in the little things on a car usually means that previous owners also neglected major things too.

Hope this helps,
Lance