If you want to go with a laptop, I would agree with the opinions that say leave it in it's original case.
It's true that much of a laptops components are held in place by the plastic case. Removing it from the case makes it much less able to deal with the vibration and impacts it will sustain during in car use.
I would advise you to look for a docking station for your laptop as well. That way, you only have to crack open the docking station to wire in a power button for the car, leaving the laptop intact.
While taking a laptop apart is entirely possible, it is much easier to muck it up than an atx style case.
A laptop install can be a good way for someone just starting off in the hobby to get thier feet wet. Everything you need for a basic system is there. You can even do without the external sound card if you wish. Just don't expect stellar graphics (yes, I play games on mine while waiting for the wife at the mall

) or sound out of it.
My advice on components:
1. Laptop - your choice
2. Docking station - for your laptop to hack in a power button
3. Airline adapter (12v adapter for laptops)
4. GPS reciever - allows you to run gps navigation and is transferable if you upgrade later (get a usb one)
5. Touch screen - This one is going to be a little bit longer:
Select a good one and do this part of the install properly. Your prior auto/stereo fabbing skills come into play here. And if you choose to upgrade to a better system later, this component is already done and you simply plug in the vga cable from the new system. This is the main interface between you and the 'puter, looking at a crappy or washed out monitor all day is just going to get you PO'd. I would recommend saving your pennies, biting the bullet and getting a transflective right from the get go. I didn't, but wish I had. Now I've paid for a regular touchscreen and I'll be spending the additonal cash on a new touchscreen, effectively adding a 2 hundred dollar increase to an already expensive item.
Regular monitors are nearly useless in full daylight and cold temps only make this worse. Transflectives are expensive, but worth the money if you want to be able to use it all the time without compromises.
Recommeded Components if you decide to stick with a laptop instead of a dedicated system:
1. external sound card - for better sound. Something like a USB audigy 2 nx can be had fairly cheaply and does a decent job for an external sound card. if you choose to get in to "tuning", I believe that card will support the KXproject drivers (I could be wrong, check it out if you want

).
2. Laptop shutdown controller- not so much for the shut down, but for the "auto start" features!
This is the basics of a decent laptop system.
Keep the laptop battery as well. It will allow you to set the shutdown preferences in xp (IE:shutdown or hibernate after running on battery power for "X" minutes) and will allow your laptop to survive engine starting voltage drops without the need for an elaborate "tank circuit' or some other such scheme.
Also, if you have
any inclination to upgrade to a dedicated, permanently mounted system in the future;
Try to buy components that can be reused from the laptop install an limit your "laptop only" specific component purchases. It'll save you cash in the long run.
One other question:
are you intending to keep your car's Head unit or remove it to mount the monitor and run the laptop sound output to the amps? This changes quite a few things as well......
There's much more advice about laptops on the forums. Use the search function to find it.
Good luck