I've built a couple.
The best thing you could do for your self is to get a solid state HD. It's sorta like a great big memory card for PC applications. You could use one designed for a laptop as they are built to absorb some of the shock caused by transporting a PC. But there is a big difference between what a normal laptop experences and having one in a car.
However, keep in mind that the read speeds of the drive will be about the same a regular HD, but write speeds are typically slower.
Heat will also be a concern. VIA makes a MOBO with a built in processor that will run XP well, and generates little heat compared to to a regular PC.
You don't need to mod up a touch screen, you can buy a 7" liliput touch screen and mount it just about anywhere. The last one I did went into the sun visor. There is a screen put out by Pyle i believe, that comes mounted to a sun visor, but it isn't touch.
Gps, OBD II virtual dashboard, MP3s through the stock stereo, etc..All of these are good reasons for having a PC in your car. The end result of all of this is simple, Personal preference.
You could use a Laptop if you have one and if you don't mind leaving an access point open to hook up when you get in, and toting around one whenever you drive. Or if you are challenged or intimidated by the install, this may be your best bet.
If however you can route wiring competently have a little PC building experience. IMO Carputers are the best bet.
As permanent as you want it to be. Nothing to carry with you. Cool factor is very high. And if you're clever about the placement of the components, it doesn't increase the theft potential. None of the installs I have done are obvious. The cars looked completly stock from outside the car until you need to load a disc of flipped down the screen. Again, very, very cool.
Hope this answers your questions, if you have any others shoot me a PM or an email. I'll be happy to help in anyway I can.
Michal
edit: lol, sorry just re-read your OP. In my experience in PCs in general both home and auto, building your own is always cheaper. My home rig cost me about $950 to build, but you couldn't touch it form a manufacturer for less than $3,000.
