I'm still in the planning stages myself for my car PC. But I'm planning on using an old laptop that I have lying around. I don't see any problem with that. The idea is to be able to use the PC for media, navigation, OBDII diagnostics/monitoring and maybe videos. My old laptop will handle all that no problem so I don't see a reason to build a new PC when I've got what I need just lying around .
I'm gonna go ahead and recommend that you use the laptop and go with a touch screen. And you can get cheap cigarette lighter adapters for laptops as well so you don't need to rely on an inverter. You get so much more out of it that just being able to watch videos. But that's just my 2 cents.
Also, this might not be the best place for your question. This is "Show off your projects", for completed projects. The forums have lots of discussions pertaining to laptop vs full pc for a car pc, etc.