if you go for the volvo and it's petrol then if its around 2001-2003/4 then they have throttle problems regarding magnetti marelli shite throttle potentiometres which used carbon or something which is prone to wear and causing issues, it wont cause out of control acceleration but if it's always driven about half throttle then it'll wear more there so when moving the throttle about that point it'd sort of jump so you'd get light throttle then quite heavy throttle - its the "ETM" issue where the volvo dealers replaced them with a contribution from the customer - has a yellow sticker on the throttle body if its been replaced but wether said sticker will be black covered in oil or if its peeled off is another matter but something to look out for, also if its about that age then unlock and lock a few times as sometimes door lock solenoids go so the pin stays up or pops back up after locking on the fob
another issue - not sure if i affected the C70's but axle ping, everybody swaps the driveshaft bolts for the XC90 bolts which have a rubber cone but theres another way to correct the fault which i never found out what it is lol!
also if its turbo then watch out for "reduced engine performance" under heavy load which chances are is split intercooler
yes i've listed a few possible issues but i had 3 S60's in a row so ended up knowing quite alot about volvos
i only had the axle ping issue which was about a £30 fix from local volvo dealers and a torque wrench so i suppose i did quite well regarding reliability
HTH
Alex