I wouldn't recommend the use of car headlights. They are not the right spectrum and they won't give you enough light. They are also 12 volt and will require a transformer. For a 6' tank, IF you don't intend to keep any of the more demanding corals, you would be OK with either VHO
or Power Compact lighting. If you think you might want to try some of the more "light needy" corals later, you should probably go with a metal halide system to start with. There isn't really any cheap way to light a tank that size. It starts at expensive and goes to ridiculous. If you are good at building things, you might save a good deal of money that way.