def. not BMW. they're knowledgeable about all things BMW, not aftermarket. For that world, i would turn to aftermarket experts. Should be pretty easy to find a good shop, just look for the telltale signs, clean well lit stores that carry mid to upper end audio. as for installation, ya just flat out tell 'em what you want and what you have....most installers won't work on something they don't want to or don't feel comfortable with, so if they hesitate, you might look elsewhere.
installation is very simple on these cars...i've pulled the head unit and added an amp and sub to mine...one of the easiest installs i've done....and i've done a lot!
oh, btw, look around on crutchfield.com and use as a basis for pricing....they are pretty fair....i would definitely make sure wherever you go, you pay less, or at the very worse, the same price as their store. local shops around these parts generally beat them by 10-20%. hope this helps.