You mean a trailer hitch receiver? Have you searched this thread?
You can put on a 1 1/4" one only, seen in this DIY.
http://www.e90post.com/forums/showthread.php?t=57786
The electrical is a piece of cake since the battery is in the trunk, no reprogramming needed with this kit:
http://accessories.etrailer.com/search?w=119176
No drilling needed.