If size is any consideration for you, I'd go with one of the Zotac ZBOX Nano's - they have a VIA-based one and an AMD-based one. From my experience with VIA, go with AMD . Barebones AMD version can be had online for about $220 last time I checked.
As far as the power block, instead of going Interter->Power Block->Unit, get a DC-DC car regulator, like Opus, Carnetix or DCDC-USB and use that to power it directly. You would need to get a barrel plug that fits into the DC jack of the computer - RadioShack sells those for a couple of bucks.