Don't let anyone discourage you, they're just exhibiting textbook "fear of the unknown". The brake control installation is simple, and just like any other vehicle, once you know where to pick up the brake signal. (see post details above).

Do you already have the hitch installed? I installed both my hitch and my Prodigy Electronic Brake Controller at the same time, and of the two installing the hitch was the bigger/less intuitive/more complicated job.

Armed with the instructions above in this thread, doing both jobs at the same time is an easy Saturday afternoon for any competent do-it-yourselfer let alone a "professional installer".

PS - Regarding the Electronic Brake Controller installation. If you don't want to go the dealer, I would guess you would have more luck at an independent BMW shop than a "trailer hitch" specialist, since all that is required is a little familiarity with the BMW electrical system - and the "trailer hitch guys" are used to seeing American iron and "plug and play" installs.