First off, when you start puttin that thing together, we need to exchange some pics. I've been trying for over a year now to figure out a good way to install one in my 2000 Cherokee. I "think" i've finally hit upon the best solution for me at least. if your having any trouble figuring out a good way to install it, by all means lemmy know and we'll exchange ideas (outa curosity, what state/country you from?).
now, on to my (limited) thoughts on your setup so far. I'm sure some much more experienced people will have more to add.
skip the PSone screen, it wont work for what you want to do. first reason being that its too small. second reason being that its composite video. surf the net? not a chance, you wont be able to read anything. same goes for 7" lcd monitors with composite video...they're bigger, but they still wont get you the detail you need.
hate to say it, but one of the few options you got in that realm (and sadly the cheapest) is a 7" VGA touchscreen from Lilliput. you can get em off ebay for $280. you can go with a little better quality Xenarc display for $400 also.
now that you eliminated the TV-out need, you may be able to get by without getting it on the motherboard (unless you plan on giving your passengers an lcd monitor to watch movies on, in which case a composite video tft [like the PSone] will work fine).
I dont have a good link handy to a PSU for you, but there are some good DC/DC PSU's that work with the VIA mb, someone is sure to have it handy.
Is the keyboard/mouse wireless? I you go with wires, i'm guessing it wont be long before you spring for the wireless =)
on the topic of control, make sure you think thorugh exactly what you are doing, and how you want it controlled. alot of people use premade frontend programs like MediaCar and others. these offer a number of advantages. they're skinned, and look nice in your car. they also are designed for use with the touchscreens, and work quite well.
I completly understand the limited funds problem (I've having a good amount of trouble in that area myself), however, her are a couple "extra" options for you to consider in the future.
GPS - $100 will get you a GPS reciever and software.....never get lost again! I've been using one with my laptop-on-the-seat arrangment for a years now, and love it.
DVD drive - maybe its just me, but I plan on putting a slimline DVD drive (still havnt decided if I want to spring for the DVD/CDRW combo yet) in the front dash. this gives me alot of options, such as letting the passenger watch a movie or burning a cd for a friend right there.
The great thing about these computers is each one IS different. everyones got a slightly different idea of what they want to get out of their car-puter, and builds it to meet those needs.
well, I think i've rambled on for more than long enough, just a little something to hopefully get you thinking thorugh some of the ideas I didnt think to think about untill later.