Sounds neat. Don't short figures when putting together estimates, remember that every piece of commercial software needs to be licensed (including the OS), and before you get too far, you should probably contact the authors of any share/freeware stuff you think you'd install and work out an arrangement with them. This is an aspect of carputers that is often overlooked. Don't assume that just because it's free for you to use Frodoplayer, that this automatically translates to it being ok for you to sell a carputer setup with his software installed. Assume nothing, contact everyone, and make sure your ducks are all in order.
Also, remember not to shortchange your time. That $5,000 figure isn't just the $800 in hardware, it's also a lot of reasonable hourly rate for installation, configuration, etc. Based on what I make at work, I've put a few thousand dollars in 'labor' into my car by myself, and my setup isn't anywhere near as fancy as a lot of the others.