I agree with Daox. You should start with a small, cheap and fuel efficient car for the beginning. With some mods you should easily achieve 50+ MPG. Then you'll see how much money you're saving for your project.
I've changed Toyota Previa for BMW e36 Compact diesel just to have enough money for all the mods I'd like to do ;-) Now I can get 60+ MPG so it was worth the effort
