So you want to keep on eye on your car, do ya? sounds cool, no pun intended. I too have been thinking about various remote monitoring setups. I'm assuming a few things here, are you monitoring the vehicle or from the vehicle?, Budget? input(i.e. monitor, keyboard) needed?
You could approach this from a few directions:
1st, off the shelf: one could use a full featured cam plus a notebook and send the data stream to a web page if your able to have a wireless connection, use a cooler in the backseat with notebook, put a car fan on the camcorder, and get one of those window mounted solar fans to help keep the car ventilated. put in a few extra cig.outlets and a few adapters for running the notebook/cooler/fan/cam and your happening without a big build.
2nd, minor build: pick the smallest, slowest CPU, MB that will drive your software, keep you power/heat footprint as small as possible. you should not need stuff like, high end sound, 4GHz or high end GPU's. maybe you can get away with high end passive cooling on slow/low power devices.
3rd, major build: you could forgo the power adapters and put a few marine batteries in the trunk, I think you could get quite a few hours off them running just those 3-items. Put in remote start on the vehicle and wire batt's in with car and let the car charge 'em when needed. Or, here's a good idea, on a van put a solar panel on the roof and charge batt.'s.
Some side notes: some IR equipment is sensitive to local temp.s. also if the temp. and humidity vary much you'll have to address keeping the lenses clear.
Have you considered sending the signal to a secure location, where you don't have to fight the elements so much. minimize your equipment exposure.
Let me know what you think,