Suspension

I have built one with independant suspension and unless you're going to drive like Stacey Kohut, I'd settle for a little elasticity in the axle or a pivoting axle so it behaves more like a 3 wheeler.

Suspension is complicated to make, shocks are expensive and do nothing for aerodynamics unless you hide them inside bodywork. 

Quite a few carties have suspension, but I suspect that for the most part that is there simply because it looks cool. It's difficult to get right, and can cause some serious handling problems if it's done wrong. A common mistake is to make it too soft, which can introduce all sorts of very unpleasant handling issues at speed. That said, however, relying on chassis flex can be equally dangerous as well.

It needn't be expensive. Midi moto shock absorber units can be acquired in ebay for around £10 each and work perfectly well, although you'll probably want to wind the preload up as high as you can go.