1st. Get your hardware sorted out - What computer, what accessories, how are you going to mount it, how are you going to connect to your sound system
2nd. Play with the various front ends. Road Runner is one of the best, but it also takes a little tinkering to get it to work how you want
3rd. Set up your system in the house, with all peripherals attached, and get your front end working correctly
4th. Install in car and check if it all works. If not, repeat step 3. In this case, a laptop seems to be the simpler solution.
Cheers and good luck