I think the best alternative energy car would be a hydrogen combustion engine.
Hydrogen fuel cell cars are very expensive due to the use of platinum. However, Combustion engine components are cheap and already mass produced. Conversion to Hydrogen Combustion is also cost effective, as you really only have to replace the valve and valve seats with chromed hardened metal (for a good seal as Hydrogen is a very thin gas), and replace the fuel injection system with a gas fuel injection. (Or carburetion).