I'm not sure how much research you've done on the hardware/software side of it, but there are a couple of different options. Either buy a handled GPS unit that has a USB output to connect to the laptop (or bluetooth) or they also make GPS PCMCIA cards if you want everything to be totally integrated into the laptop.

search for "GPS on Laptop" in Google and its all right there.

You using a power inverter for power?

Here's an off the wall thing for you to think about though. If you want this to be more permanent, you can get very small form factor towers that could fit under a seat, then permanently mount a flip down small LCD from the roof and use an RF keyboard with built in mouse. Though I admit using an already existing laptop would be easier