Well, first, a fair warning, some people are probably going to yell at you about not searching the forums, because these questions have all been answered already. But here is a quick rundown of answers:
1) Many people (including myself) use the M10000, and have no conflicts with windows xp, it is perfect for car-pc use
2) Many people also use the Lilliput, and it seems to be about the best value you can get, check out www.digitalww.com
3) Many people use the Opus, and swear by it. I use an inverter setup, because of the price of an Opus.
4) Us Carputer people are from all over the world, from here in the U.S. to India, to UK, to Australia.
Also, make sure you look at all of the cars in the "Show off your projects" Forum, it will give you ideas and answer some questions