definitely lower the car. an underbody tray may or may not help a lowered car. rear wheel fairings are good - although I may have found a much better solution
imho your mpg speculation is on the high side. can you get a smaller engine? less weight, and not so much excess torque. you'll need <15 hp to maintain 60 mph. perhaps a modded vw jetta tdi (54 mpg stock).
you could get away with 205 LRR tires.