Well if you are looking for an aftermarket headunit that can double as a GPS. I recommend the Pioneer AVIC-N1. It is a nice size screen, for a decent price, and has awsome features such as a full Dynamics display that can tell your 0-60 or the amount of G's your pulling in your car. If you want to keep it stock, I would look on ebay or something. Ask the audio forum.
Dinan Stg 2 Phoenix Yellow M3