I may be shunned for saying this, but here is an option. I know you have mentioned you are going with Alpine for the GPS functionality you need, but have you looked at Kenwood. Here is what I propose for you (of course maybe that sports car would sound better):

KNA-DV2100 - GPS System
KVT-M700 - 7" Touchscreen and receiver.
Music Keg - 20GB for your MP3's

So a few months ago I asked Kenwood about the Nav system. It must be mounted horizontally, not vertically. So you put the touchscreen receiver in the dash, the nav system and keg in the trunk. It shouldn't take up too much of your precious space. All are controllable through the touchscreen. Total is about $4500 for all 3 though.

I still love my empeg though! Best of luck on what ever you choose to do!