Sounds like you have the basic idea down pretty good.
Here's what you'd need...
Firstly, to get sound from the computer to the head unit, you'd need to find out what kind of AUX input your head unit has. If it has a 3.5mm jack (the regular speaker output jack size), that's about as easy as it gets. Simply run a 3.5mm audio cable from the laptop to the input jack on the head unit. At this point, audio would be done. IF it doesn't have a 3.5mm jack, you'll need to find out what it does have. You may have to buy an adapter to go from a 3.5mm jack to RCA's or some other form of audio cable. Easy enough to find on the net.
Next, you need to do video to the touchscreen and USB to the screen. For this, just get whatever touch screen you want (I recommend the Lilliput EBY701 if you want to go cost effective). Make sure your laptop has a VGA output. If it does, then simply get a long VGA cord (10ft or so) and then run it from the laptop to the touchscreen VGA input and configure your laptop to use an external display. For the USB cable, that's simple as well. Just get a long USB cable (again 10ft or so) and run it from the laptop to the USB input on the touch screen.
Lastly just figure out how to power the laptop and you're good to go.
Later on you can do more research on software and hardware mods. Just keep it simple at first, then expand once you have a grasp on the CarPC as a whole.