Have you considered a Pioneer AVIC or Kenwood equivalent (or any of the others)??
Otherwise, a few points:
A laptop harddrive as it's designed to deal with bumps/knocks better.
What makes this job really easy it so just flat out use a laptop, low power comsumption, deals with heat, deals with knocks, can get car power adaptors easily, is small etc etc etc. Consider what a cheapy laptop is worth these days vs what it would cost you to build all the bits you will need. Second hand they are dirt cheap, even new you can find them at $550 on special.
For power (as it looks like your doing anyway) it's cheaper and easier to go from 12V -> 110/240V -> PSU (12V) than to run straight of 12V.
USB has a limited range. Consider any controls (including the screens) will be limited by how far away you can put the computer and still use them.
Wireless is possible and can then be used for syncing, Bluetooth or USB GPS is possible and easy to set up. Syncing with your phone is possible, pretty much anything is possible.
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