I have a 750 which is comparable(although newer). First, find a shop with trained BMW mechanics. DO NOT take it to a dealer. Repair costs are twice as high at my dealer than the shop I found. Mine has been pretty maintance free so far(knock on wood!). I had to replace rotors already and tires are coming up. Oil changes are a bit more expensive, but you only have to do them every 10K miles. I love my car. It is the nicest vehicle I have owned. I came from a history of Cadillacs, and they just don't compare in ride quality and handling. Make sure all scheduled maint. has been performed on the one you are looking at and have it checked out by a mechanic before purchase.
Also, if you can find an M5, do that instead.