Welcome to the forums
Most of your questions have been asked, and answered in the past month or so!
Firstly, cheap and good do not go together in the same sentence (apart from that one, or the sentence "cheap displays are rarely any good"). If you want GPS, you want VGA.
Also, check the (current) latest thread in the GPS forum (hint, it's to do with Linux).
Many people remove their head units out entirely, and run the audio outputs directly into an amp, and control the volume via software on the PC.
I personally have not seen any good xbox integrations, but on the extreme, putting a full-sized tower is also not a good idea. I suggest one of the Via Epia motherboards.
For power, you will probably be best suited with an Opus power supply. There is plenty of information on this forum about the power supply.