1) How would I make a LCD into a touchscreen, or am I completely off? I was reading some posts and they said that making a touch screen would be much cheaper.
Cheaper than what? Also a touch screen is usually an overlay that you put on top of your LCD. generally touch screens emulate mouse movements.
3) I would have one LCD in front of the driver, which would replace the whole dash and have a program that displays speed, RPM's, amount of gas left, etc. Where could I get a program like this and how much would it cost?
Definitely not a good idea. If something goes wrong with your computer you have no gauges. You could do something in addition to, but I would NEVER replace the gauges that are supposed to be there. But what you are looking for is an ODBII adapter (odb 2)
4) Do I just connect a computer? If so, what kind would I need to do only what I want in section two.
You are going to need a signal amplifier to output to all of the seperate displays. Don't worry though, these are pretty easy to find. If you're computer has tvout it's simple enough to connect, if not you whill need a scan converter which converts your vga signal to ntsc.
5) Would it be better to view navigation, time, temperature, and GPS (what is
the cost of GPS?) on the monitor in section two, or section three?
a GPS costs anywhere from $100-$500 If you want to have your displays showing different information you are going to need 2 video sources. Which would either be 2 computers or your computer and another source. I doubt you will find it necessary for your back seat to have GPS data instead of a movie or video game.