The great thing about CarPCs is that they can be as complicated or as simple as you want them to be.
Once you get the hardware installed, the software side of things is super simple. Or, once you're into it a bit more, you can get more detailed with the tweaking. All of this information can be had on this forum. To boot, there's a ton of folks on here that are more than willing to answer any questions and to help you along in the process.
There are two routes that you as an installer can take:
The custom, one off install (built the PC, mold in the screen, etc).
The all-in-one solution like the G4 or the T3.
There are a number of options for VGA touch screens in the car. If you're going to run a PC in the car, I'd highly recomend running a VGA monitor. Because of the fonts and what nots in Windows, lower resolution monitors will make things look rather blurry.
Regarding audio....just check my signiture...I think that will answer your question about the quality. There's a few of us out there that are competing with CarPCs and are successful.
Currently, the best solution is the 701 from Alpine.
There's a group of us working on a solution, that might have been the thread you were reading.
BTW - check the title under my screen name. My job is to help you (as a dealer) understand CarPCs and how to implement them.
PM me if you have any specific questions.