Personally, I prefer to keep the stock head unit if possible, and connect the PC audio into an AUX input on the head unit. That way, all of your stock head unit functions still work (like steering wheel controls, bluetooth, FM/XM, CD player, etc).
If your stock head unit does not have an AUX input, you can usually find aftermarket adapters. I got mine from pac-audio.
Then, the tricky part is just figuring out where/how to mount a touch-screen.