I am no expert by all means, but I have learned quite a lot and spend hours reading here. A tablet PC will work but it may not be practical for you. If you want a clean looking install, then you will have to find a mount for it. However problem is, it doesnt look like there are mounting points on the back of a say P600
Anyway, of course you can lay it on your passenger seat but you will have coutless wires all over the place based on your description. They do sell Carputer kits on ebay in case you havent looked. I dont think your biggest concern should be the "pen battery" since you can easily find them on ebay.
Regarding OS's. I would stick with Windows. Easier with drivers, more accessiblity to programs, etc. iTunes sucks and is resource hungry, there are plently of other music playback programs (winamp, etc).
You say you want a interface that is touch driven. Do a search on Road Runner. I am using it and it is fantastic and easily customizable.
If you have expenable money and can afford, on top of your cellphone plan, a PCMCIA cellular data card then most will do just fine, they just tend to be expensive. And you have ATT and they are most expensive. Either way it can work. I have Cricket Wireless in my area and they just rolled out with their wireless internet PCMCIA cards. For 30.00 bucks a month I have internet everywhere I go in my home network. Love it.
Anyway to be honest, I would look into another route instead of a Tablet PC. But if that is what you have your heart set on then go for it. Just make sure you keep a clean install. I have seen some installs here that are pretty horendous, with wires and such all over. I have always understood Carputers to be more or less stationary in ones car.