Solscud007 - One thing to keep in mind, a car never supplies a steady 12 volt signal from start up (as I've read and somewhat experiences), so make sure your friend realizes that it's not enough to do a straight dc - dc powersupply in your car, it has to have a tolerance anywhere from about 7-15 volts..
there are a few psu's around (and instructions for building) that have tolerances from 5v -15v
I'm planning on doing the same thing in my brother's car this summer, under seat installation as to leave him with as much trunk space as I can and to deter any possible effects of the large magnets and increased shocks from pounding away (he's got 2 12" type R's in there). I'll be using a laptop hdd, and possibly a combo or just slimline dvd drive, luckily we've done all the installation ourselves so makes things easy to set up. One thing we got goin for us is that my brother bought a kenwood touch screen in dash monitor with about 3 inputs so we can use that for output, i plan on this playing dvd's, divx, and mp3's with removable hdd tray, and all based on linux, for effeciency (start up time, etc), best cpu usage, etc.