My 97 s90 is the first and last Volvo I will probably ever buy. Not because its a bad car, but I cant see me buying a FWD volvo with the problems they seem to have. With that in mind,watch out for the timing belt. If you dont have documentation that it was done previously, do it right away. We have had our car since it had 16k miles on it, now it is approaching 175k. Had a few things go wrong, replaced the radiator about a year ago, had to fix the ac controller module (resoldered it myself) and have had to deal with the biodegradable plastic they tend to use throughout the car. Have replaced rotors and brakes a couple of times. Still on the original shocks. Ac worked on about 3 years ago by the dealer. Mostly odds and ends type jobs, nothing really major. Highest dealer cost was about 600 for an oil leak that involved removing the intake manifold.

Never had problems with the sunroof. First set of rotors went on at 90k miles. Front end only had some rubber bushings replaced about a year ago, but it sounds like some more are starting to make noise. PNP switch likes to act up every year or so. I row the shifter back and forth 30 times and that wipes the contacts clean for awhile. Probably the best thing you can do is become familiar with the car and do your own maintenance. I never have taken it back for the "prescribed maintenance" like the owners manual calls for. Guess I cant be doing to bad for the miles that are on it. Good luck.
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97 S90 with 173K miles still going strong