I thought about laptop route too, knowing that a laptop with broken LCD screen can be had for $50, or if you know where to go, you can get PAID by taking broken laptops away from some people.
But more you think about it, it's easier and cheaper overall to have a Desktop PC mounted in the trunk or something.
Common perception is, that Laptops are better for harsh environment mobile application etc etc, but in-car PC is not really a mobile computing, because there is a constant power source, and weight is not really an issue.
Laptops are designed to be used without a power source. Also they are designed to be carried around by person (light, small, everything in one, etcetc.. it's the same)
CarPC HAS constant power source. Also, you don't have to "carry" laptops in your car.
Availability of Power Source is EVERYTHING that matters in computing. PERIOD.
I've seen many posts where they start with a "Laptops with broken LCD from Ebay for $50" and ended up getting a micro desktop at the end.