definatly buy one. i got an 85 635csi and the parts arn't really THAT expensive(some are tho). lots of used parts on evilbay and these cars are definatly fun to drive. be sure to get a good dash as these are really expensive, but a 5-speed is a must. the 85 has the strong trans(getrag 265) but i rerally enjoy the metalic black one i got for the wife. m-parallels from a 7 series look great on mine. as soon as i get the 300MM brakes on it i'll post up some pics.
hope this helps,