Wow, I'm about to give you a lot to go research.
It's totally worth it when you get it running because it's totally customizable and you can add things to it for pennies. If you buy a flip out DVD player from Futureshop and want to add GPS it's about $400 for the unit. If you want to add GPS to your car computer it's $50 for the USB receiver and $100 for the maps.
Audio out can be done 1 of 2 ways:
1. If you keep a traditional CD player in the dash, you can run the audio out cable from your computer to the RCA input on the back of the headunit. If you want to play a CD like normal, just pop it in. If you want to listen to music from the computer, switch your headunit to AUX and voila. The signal is amplified by the headunit so that's all you need to do. The major problem with this is trying to put a headunit and a touchscreen in your dash.
2. If you don't keep your regular CD player like myself, the audio can come out of the computer and into a regular 2/4 channel amplifier. Then from there out to your speakers, but either way the signal from the computer has to be amplified before hitting the speakers. And some people prefer to pre-amp before hitting the amplifier however that's another issue.
Most computers I know of are powered by a DC-DC power supply which takes a 12 volt signal straight from the battery. These have many benefits like powering down so they don't drain the battery, startup/shutdown controllers and less install space. You can use an inverter if you want but they don't have startup/shutdown controllers so your computer won't know when your truck turns on/off. You'll have to manually reach down and hit the power button each time you get in/out of your truck and that will suck big time. Or you can use a custom controller but why not just spend the $60 on a DC-DC power supply? If you use an inverter you'll be taking the DC signal from the truck, using the inverter to convert it to AC signal, sending it to the computer power supply where it gets changed back to a DC signal and then powers the computer. Thus you get signal degradation.
If you don't use a touchscreen.....well.....I pity you. some people prefer the touchpad mouse or thumb mouse, I don't know how they do it. It's just much easier to have the buttons on the screen and you touch what you want. Plus your passenger who wants to choose music knows how to use it instead of trying to figure out your mouse setup.
Right now there are sunlight readable and non sunlight readable, the difference is about $300 between them. Personally I just bought a Xenarc which has a 1000 NIT and simply tries to outshine the sun instead of using the sunlight readable technology. It's bearable for me. If you don't go sunlight readable, the brands are pretty much Lilliput and Xenarc and from what I see on the forums the Lilliput is kind of a cheaper make which is why it's a cheaper cost. I chose the Xenarc because it still works at -40 degrees celsius here where people say the Lilliputs crap out. It also has automatic backup camera switching and stuff, you really need to research the features of each. MP3CAR has their own line of monitors as well but I can't tell if they're just rebranded Lilliputs or what they are.