Welcome to mp3car.com!
I built a windows8 carpc with USB DAC, using the Objective DAC board feeding an MS-8 processor. It has a 2vrms line out at >3KOhms, so is perfect for transparent audio. There are several Android tablets running Jellybean that also support USB DAC's, that will fit into your car too.
I'm building a new carpc right now, which has windows 7 but boots in <10 seconds, using a Jetway NF9G-QM77 Intel Socket G2 Core Mobile Mini-ITX board with a i7 Mobile I7-3612QM 35Watt laptop pull I got cheap off ebay. I put a 256GB SSD and 4GB of high temp rated memory in a Voom2PC case. For DAC I used the Hifimediy Sabre USB DAC ES9023+Tenor TE7022, 96khz/24bit, 96/24 +usb to optical that i got off ebay for $45, and it's as good as the ODAC, with a fast USB receiver so FLAC plays seamlessly.
If you built something like this, and put it in the trunk, then ran a USB cable and HDMI cable to your screen, you would be set! Your sound quality would be better than most of the highest end car receivers can produce.
Here is a link to my first carpc build with the ODAC, so familiarize yourself with what is involved, and the people here on the forum love to offer help with problems you may encounter.