From the sounds of it you definatley want to go down the "carputer" route, and if thats the case you have hundreds of options!
One the one hand, you can homebrew something in linux using one of many embedded platforms, but tbh, I wouldnt recommend it unless your up for some serious work
When you think about it, what you actually want from a device in your dash when your @ crusing speed is something responsive, easy to navigate that does not need you to take your eyes of the road. None of these factors sit well with any *nix distribution out of the box, even ubunto derivitives :P That said, its not impossible, so feel free to give it a whirl and let me know how you get on, as I am also looking into these options.
Next off, you can go the "pay cash for something good" route. On one end of the scale you have the £1000+ Pioneer and Alpine offerngs, so many other AO'ers on here like, or, if like me your a cheapskate, or as I prefer to put it "bargain hunter" check out whats on offer from China. I always use Lightinthebox.com
for my chineese hardware as its straight from the hookie manufactrers and typically a perfect clone of the £1000+ option for £200-400 instead
They have plenty of options in the carputer realm that will do the job nicely!
Plan C however, is to use an embedded platform + touchscren + a decent opperating system that is easy to navigate and has plenty of applications ready to go, such as Android. As Android is infact yet another *nix deriviative, its not a million miles from Plan A, but shortcuts the amount of development work you would have to do to make it usable at speed. Another option for Plan C is to use one of the many already written carputer opperating systems (just google it to find plenty of options).
In any respect, it does sound like you want more than a noce GUI with some ODB data flashing up, so carputers would be the best way to go. As I only got my alfa a few weeks ago, I am still likling the Blue & Me (i feel it will wear off soon though) so for now, all of my embedded projects are out of the car. Incidenatlly I am building a system based on one of those PicoITX boards with the 4.8" OLED touchscreen this weekend to support another project, so if your interested, i'll let you know how they fair as a bit of kit.