Why do you need suspension? adds a lot of weight and complexity. Why not use relatively big tires? Overall every bit of weight you add decreases your payload potential. and there is no way this thing can be used on anything but a smooth road based on size and weight

If you end up with a 300 pound velo and a 300 pound payload, and a 180 pound driver you are at 780 pounds gross weight. That is a lot of weight to pedal and bring up to any sort of speed. More importantly that is lot of weight to stop, especially if you are on any sort of downhill. Will your planned braking system handle stopping 780 pounds going 15 mph on a downhill?