If you only want to listen to music and gps without going through the hassles of setting up a carputer (yes it's a pita to setup a carputer, especially for a newb), get the pioneer nav radio. Does everything you want as stated. It'll even allow a backup camera if you're so inclinded.
http://www.pioneerelectronics.com/pn...219118,00.html
The rest of it is pretty standard audio install practices (amps, sub's components, etc).
A good local audio shop can help you out there.......
Carpc's are more of a "hobbiest" thing...........
Now, if you want unparalelled control over the sound system, near unlimited expandability, have deep pockets and lots of time to waste:
Come on in! The water's fine................