This is probably a more focused question than people here are able to answer. I'd try a forum like bimmerforums.com.
With any recent BMW, I'd make sure that the six doesn't provide all the performance you need, and then some. A 535i or even a 528i (with the four-cylinder) is quite quick, and far more economical than the V8. The main downside to the four is that it isn't as smooth and doesn't sound as good as the others.