Dont buy one if you are interseted in gas milage or 0-60 times. They are grand tourers.. as such they are meant for long cruises at high speed in luxury and comfort.
the motor is kinda a hybrid in a way, it uses 2 DME's that talk to each other in a way, and has basically two m20 motors merged together. The heads are different by overall thats the basic idea of the engine. Lots of parts to break, but mostly what I have seen are electrical problems, not mechanical ones.
Having said all that, the 850CI is pretty much my favorite bmw ever. Given a little more performance I'd say its pretty much perfect in almost every way.