In 3 years the 2008 E9X is going to make a monster track car.
My advice is to go to a SCCA or NASA event and talk to people. You will see a very wide range of track/TT/Race prepped vehicles and it will open your eyes to the possibilities. I recently got to drive a student's S2000 (the track version with lots of rear aero), and it was the most fun I've had this side of a true race car.
Another thing, it's always cheaper, especially in this economy, to buy a track car than to build one. A very good source of info is in Bimmerforums, you will find an entire thread devoted to people documenting their builds.