I installed a car pc a few years ago.
I wouldn't do it again.
I had a base mini base unit in the boot, a nifty 12v PSU. A double din touchscreen. This was connected to a 5 channel amp with 4 speakers and 1 sub. I spent lots on getting the components and installation.
I had visions of GPS, Gigs of MP3s, videos, internet etc.
When it worked, it was fine, but ended up just being for MP3s.
At the time, it took time for the PC to boot up. It was useless for shorter journeys. I had to buy a remote power button to turn / reboot the PC as I needed.
These days, I have a head unit with USB reader, an external USB mini hard drive and a Garmin GPS unit and an android phone for mobile internet. That seems to be all I need.
I'm sure with mini ITX, SSD and better quality head units it would be a better experience, but I'd rather spend the money on a iPad or Android tablet that I could take on car journeys.
I hope this had provided some insight..
What are your reasons for wanting this setup?