In addition to all that has been said above.... If you plan to have lights on whatever you are towing, you will want to invest in the OEM wiring harness, which plugs and plays.
An after market hitch installer WILL NOT KNOW HOW to TAP INTO THE BMW wiring system, and may damage it trying to do it.

Therefore you will have to purchased the wiring harness anyway- why not just go ahead and get the correct hitch too?