If money wasn't an object, gold leaf. Have fun working with it though.
Otherwise, I used to use a fake gold leaf, it was called gold aluminum I think. It's slightly thicker and it doesn't have a strong gold color like gold leaf, but it's quite a bit cheaper in volume. It would still be costly to cover an entire cab in it though.
Then there's foil that's gold colored. I don't know where else it's used, but it's used often in the theatre to create faux gold props. Don't think you can get it in sheets wide enough to hide the inevitable seams.
Paint would work, depending on what you're trying to accomplish. If you want the crystalized look, Paint would work. If you want the metallic look, paint would not work. The light reflection is all wrong. If you decide to paint, use automotive paints. Slightly higher cost, but applied correctly and with a good clear coat, nothing I've seen at Home Depot beats it.