Here is my .05 worth. I too installed the Draw Tite with the support arm. The install took two of us about 3.5 hours but 2 hours of that was scratching our heads trying to figure out how to remove the rear bumper. The actual install is easy with the right tools. I think I wrote a bumpber removal thread but if you can't find it I know one exists.

I also used the wiring harness from E-Trailier and that was about a 45 min install. I did it months after the hitch install and it would not take long if you did it when you install the hitch.

Your M3 stripped down for the track is probably around 3000 lbs, your trailer maybe another 2K to 3K depending on your trailer. The Draw Tite is rated at 6000 lbs but I personally would not tow that much with my X5. It is too mountainous in my area and I would not feel comfortable towing that much weight with the X5. If I were close to the track and the drive not to demanding I would not hesitate pulling a trailer with an M3 on board with my X5. But the X5 is a mid size SUV that was not designed as a tow vehicle. You have to think about stopping and also consider the added strain on the tranny. A new tranny for the X5 will cost you more than you could by an older pick-up truck for so if I had the room I would probably buy an old pick up and save the X5 for what it was designed to do.

Lots of useful info on this board and tons of threads on hauling and hitch installs. Good luck.