vfinterceptor

The on board sound card will suck on a laptop, any laptop. Add a usb sound card to your list.
Power supply, you'll need an inverter or a 12 adapter made for your laptop. Unless you think the batery will last you long enough (trust me it won't, doesn't for me).
Get a docking port so you can hack into it for the power on/off button, unless you want to open and close the trunk evertime you want to turn it on. Or mount it under a seat.
Probably need a hub if you plan touchscreen, gps sensor, usb sound card, usb keyboard, bluetooth dongle, etc.

Also remember you're looking at very limited upgradability with a laptop. This can be a pain if you want to add things like slot load dvd's, more ram, better sound, better video, etc. And any upgrades you can find will be more expensive than a "desktop" component based system (ram, etc)

I went with a laptop because I had most of this stuff hanging around, but I would (and will) build a dedicated system when I get the cash.

Of course, don't forget the four (or more) channel amp you'll need for sound.

For advantages,
I can remove the whole system by releasing the docking port and I can work on it in the house (good for extreme winter cold issues also).
Heat issues are mostly already dealt with from the factory.
Battery backup and extended hibernation are possible.
Usually more tolerant of vibration issues in the car.

that's all just off the top of my head

I'd say if you already have the laptop it would be less expensive. But if you have to buy one, probably more expensive.

Just my .02

Cheers

BJ