All I'd like to mention is take a look at the other stories of "buddy" or "cheap" installs on this very board and others, and understand that you get what you pay for every time. You've got a CAN network in that car and special care needs to be taken not to damage it during installation. There is also a special interface that is far more than a wiring harness. You may also have OEM amplifiers that need to be integrated properly.
Keep all of these things in mind as you search for a shadetree "installer" to work on your nice German car, and understand that repairs will cost FAR FAR more than a proper installation.