As far as materials, chromoly may be able to be designed to half the thickness/weight of mild steel, and may well be similar to at least some aluminum components, 6061? 7xxx?
If you wish to be minimalistic, and efficiency oriented, then anything you can do to keep the weight down will help. Your design of say a 500 pound car will be very different than a 1500 pound car. And, the heavier you make one component, that will trickle down through the rest of the vehicle. Stronger frame, stronger motor, stronger wheels, etc.. and more weight.
I applaud your idea of building the car with hardware store components, bolt on and pop rivets, but I'm not convinced that will achieve the sleek car in the photos above.
Kit Car?