I have a 328 which i bought 4 yrs ago. Basically I have not had much problems except for the normal age related issues. For the first 2-3 yrs, it was great, real low maintenance. Now stuff are starting to take its toll, so in this yr alone, have changed the radiator, TDC sensor, camshaft sensor, brake pads, ball and control joints, absorbers, power steering hose and tank, rubber strips for the windscreens. Also later on have to look at replacing the rubber strips on the door handles and prob the rubber seals on the doors, cos the wind noise is getting louder. Other than that the only thing to look out for is the gear box (cos of age again) and the electricals - they will be wearing down. Engine wise, it would be hard to break it. Also there are now some traces of leak fr the oil sump, not too serious yet but will have to get it replaced at the next service interval, which should be soon.
Other than that, its a great and fun car to drive.