I can help you a lot with the custom linux interface
As for power supplies, it's an interesting issue
It comes down to this:
Do you want to get a DC->DC power supply?
If you do, you will need to figure out how to wire an ATX power supply to the G3. To get a 150W DC/DC ATX power supply, you'll be looking at probably $180 (including something to clean the power).
Or do you want a new AC->DC power supply, in addition to a high performance DC/AC converter? (You'll want to make sure that it provides a high enough wattage, and that it's clean).
Again, you'll probably have to figure out how to get an ATX supply wired into the G3, but you're looking more towards <$100 (though efficiency will go down and it will draw more juice from the engine).
Also to worry about is crank power (you won't be able to run the computer while cranking the engine), and how to power on and off the computer.
Idealy you'll have an extra big battery to supplement the power comming from the car's main electrical system so that durring cranking or whatever you can provide uninterupted power to the computer.
I have quite a bit of experience researching all of this stuff, so if you need anything else, just ask.