I'd just get a UMPC and build in a good mount for it.
That's easy to steal and power heavy as well as not hardwired into the car electric system, so there are conversions to keep track of. Also most UMPC have very weak processing power of videos especially, nor to they use good quality fast SSD that I am looking to spec. Most UMPC don't have a touch screen, either. And for the price of a good performing UMPC, I can build something that suits my needs and is faster.
I also want it to look relatively OEM. A mount with a UMPC tacked on does not look OEM. It would also take up space in the passenger area. My idea with a retractable touch screen and single DIN computer would hide everything inside the dash, making it disappear from sight when not in use and fully utilizable by both front passengers when in use.