Here are your questions with my answers:
1) How would I get radio from an ITX computer? This is my biggest problem so far.
A: You will need to get a FM car with an antenna. Do a search or go to www.hossweb.com and look at his expedition that has all this on there.
2) will removing the airbag and/or fuse light up a dummy light? How do you get around that? if no way, I guess I could use the glove box, or mount it in my trunk and run cables.
A: I would suggest the trunk and run cables. It will be the safest back there then taking out an airbag.
3) Dash kits. Do they make dash kits for LCD screens? Will the PSone LCD screen fit in my dash?
A: Not that I know of yet. there is not that big of a market for this yet and all jobs are custom do it yourself jobs. I would not attempt this if you dont have a clue about customizing your LCD screen.
4) Sound. Can I reuse my old radio some how, or will I just have to buy an amp. all I have are the 4 stock speakers, no subs or anything.
A: There are many setups that you can do. the cheaper you make your setup the cheaper your sound will be.
5) ODB-II. I just briefly read this topic before posting here. This sounds outstanding! How would you interface the computer with your car's ECU? I see there's software to read it. This makes me want a carpc even more!
A: I would suggest some extensive reading as there is tons of information on this. just go to the ODB-II section as if you were going to run this on a laptop and make it a permanent optoin for the carputer.
6) Parts suppliers. Where can I find some of these accessories for cheaper? IE: PSU, the automatic power up/power down with your ignition device .. cables, etc..
A: you should not be asking these questions as you should READ and all these questions would have been answered.
I would suggest going to www.hossweb.com and read all three pages about his setup and you will learn alot. he also has a video he made. do that and then ask questions!