I had a 1998 S70 T5 (HPT) 4spd auto. I had 3 problems. First the air pump went out 2 days after buying it used. Air pump just recycles exhaust gas back through the engine for efficiency when starting cold. I had it replaced by dealer at my expense, and it failed again. Apparently the check valve was stuck open causing the air pump to try to pump condensation. I ran the car over a year without the pump like this and check engine light on, didn't seem to have an issue other than the check engine light. My steering rack began leaking and it made turning diffcult, got that replaced. Also my ABS braking module failed which I got repaired rather cheap due to my location in Washington State. I had the car for 2-3 years and enjoyed the turbo, lag is present but it can be somewhat mitigated with bolt ons. I drove the car from about 70K miles until trading it in at about 125K miles. I also had a 2005 Volvo S40 2.5 liter 5 cyl (version 2) that had air conditioning problems. Currently I have a 2009 C30 T5 R-design 6spd manual and haven't had any problems with it, up to 54K miles (got it used with 13K miles in 2010).
Volvo parts aren't cheap at least at dealerships. Repairs can be done yourself but you'll have to peruse forums or find a knowledgable friend. Oh and don't get an S60R or V70R as a daily driver.