Go for it. The Mini is a great computer for a carputer. It's really sad to read people's worklogs and see how much time they spend building a small computer. They don't end up saving any money over the cost of a Mini and they usually spend more. You're going to have lots of other problems to solve besides building the computer. Leave that to Apple and focus your efforts on customizing it to your needs.
Also, think of resale value. If you ever decide to do something different a Mac Mini will sell faster and for a whole lot more on eBay than some hacked together box.
Also, check out the Carnetix P2140 instead of the 1900.