http://www.e90post.com/forums/showthread.php?t=365866
lots of info on the system in the above threads and links
I think you need Can bus adaptors to run the steering wheel controls with your new HU
There is also some kind of terminator for the optical bus you may have do other stuff to it I'm not real sure on that.
Some people move there existing head unit to the trunk(hooked up the same) or somewhere else so there is no change that the rest of the car sees