Yes, you can have a laptop as a carputer. I currently have a Compaq Presario x1000 in my Jeep Grand Cherokee installed in a compartment in the trunk. The laptop is hooked to a 7" Xenarc touchscreen lcd I mounted onto the dashboard. The computer itself is plugged into a 160watt inverter that turns off when the car is off. My Jeep has 3 cigarette lighter power outlets and 2 of them are always on (hot) regardless of the car running or not. I originally had the inverter plugged into one of those; however, I accidently drained the battery over a weekend of not driving and leaving the laptop running. After having to call AAA for a jump, I decided it's not worth the risk of always having it plugged into a live outlet.
Basically, I put the computer on Standby mode whenever I park it for work or at home and it can keep the charge for well over a couple days; of course, it's constantly charging when the car is running, so I never have issues while I'm driving for consecutive days. This is the easiest solution for getting it to turn back on quickly. Since my laptop is out of reach from the driver's seat, it would be extremely annoying to have to open the trunk compartment to turn it back on. I have a wireless logitech mouse that I use to bring it back to life from standby mode. Beware that bluetooth mice don't have the ability to do this... at least not ones that I've owned.
When you say you're thinking of wifi, do you mean using a kyocera kr-1 router or something? I have a sprint mobile broadband card hooked into the laptop with an external antenna for better reception. I was thinking that a pc card router would be neat to have, but it's just another device that needs power from your car. You don't need it if you're just thinking of using an evdo pc card with one computer hooked to the internet.
Hope that helps,