I have a 1986 635 CSI and I absolutely love the car. Hence the reason I have put far more money into it than it is worth but what a driving machine. The biggest problem I had were the metric tires. The old 390 TRX tires were a really hard compound. They wore like iron but they were loud and bad on wet roads. Michelin stopped making the tires and the only source wanted $400 apiece for them. I bit the bullet and found 16 inch wheels from a 2002 5 Series with the same offset and put a good set of Michelin 215-55-16 Primus tires on them. What a difference. It cost way less than the TRXs and I can use standard tires now. Make sure you have a BMW mechanic check out any 6 Series before you buy it. The suspensions are complex and the shocks are expensive. You really want to make sure you go into a purchase with your eyes open.
You will love the car.