I always thought laptops were ideal for carputers. You can leave the battery in place which means you can leave it asleep since it won't draw much power.
Also they are designed to cool in a very small space. All you have to do is increase the space and you make a huge difference to the cooling. Just put it in a custom case.
You could duct air in from the front of the car for it. You'd need filters but its no big deal really. Add a couple extra small fans, and you can pull air in while you're parked without flattening the car battery.
A late TiBook would do the job quite nicely. Depending what you want to do with it. The DVI models should all run Leopard without too much work. We have a 450MHz Cube running it just fine.