The installer has to specifically set-up the system not to play while driving -- otherwise you can have it on anytime you'd like.
I'm going with an in-dash unit this summer -- Kenwood for about $1,400 (US) installed. Panasonic makes one for about $1,000 (US) installed but my buddy is co-owner of a custom shop and he raves about the Kenwood. Its the one he has in his truck. Besides, its touch screen.
I know what you mean about not letting anyone rip into your dash. Ask around and check out a few shops in your area. Ask to see pics of their work. Oh, and the BMW dealer in your area may very well know of a great custom shop.