For any kind of vehicle application I always recommend using the mini-ITX form factor systems. They aren't the cheapest you can get, but they are super power efficient. The one I'm looking at getting is the
ARTiGO Pico-ITX kit that sells for about $350USD. The only kicker is that it's coming from over seas so the shipping to the US is $98 for 5 days air which kind of hurts so I'm still considering going with a parted together system from newegg.com. Theses systems, however, if used with a solid state compact flash card as the hard drive (they make up to 32GB cards that sell for less then $100) they can consume less then 40W of power, often times even as few as 10W or less when idle.
The Pico-ITX kit doesn't have a COM port, but it comes with everything you need right out of the box which is a great bonus since parts for these systems can be hard to find or expensive if you do find them. But it has USB ports and I know they make USB-COM port adapters (at least I'm pretty sure they do).