I intend on building one of these up.
Try a HP thin client.
About 800mhz, run laptop ram (I just upgraded mine to 512 GB), and if you buy a $10 cable, it converts it from the 32mb CF card that is in it, to a 44pin female connector so you can run a laptop HDD. Mine has a 100gb. The beauty is that they run on 12v already, so all that is needed to run it in a car is a 12v DC-DC regulator (preferably with a shutdown controller). On top of all that, it's fanless!
I wanted to create a double din unit with the PC, power supply and amp, with a touchscreen TV mounted to the front, but the only amps I can find are kit amps - I'm not confident enough to solder up something like that, so it looks like it might be a no go.