Check maintenance records. If you buy, make sure to budget for any possible repairs. An 850 w/150K on it should have had the timing belt changed twice. Check the cooling system (thermo, water pump, hoses). Check the condition of the fluids: Is the brake fluid relatively clear? Power stering fluid reddish or brown? Tranny fluid reddish or brown? Check the brakes: Are the rotors scored or warped? Check tire wear, too (they can get expensive). If the exterior, interior and engine bay are clean, then it's likely the previous owner(s) took good care of the vehicle. I'd say $6,000 is too much for a car that you are not sure about, but if everything checks out, the car should be fine.

As far as the constant problems, the only thing out of the ordinary on my wife's '97 T5 wagon was that both O2 sensors went bad about 65K. I don't think you'll have constant problems as long as you keep up with Volvo's maintenance schedule (which is extensive and can be expensive if you don't know how to do it yourself).

When I was in college I had an 1983 Fiat Bertone X-19 and a 1985 Peugeot 505s. Thank God for my parents! They paid for all the repairs I couldn't do myself...