I know a brazilion people have already said this, but it is damn cool that you are making your own bodies.
Have you thought about making any open-wheeled style body that would fit onto the M18/MRS4 chassis? Maybe something like an older style F1 car (before all the stupid little winglets) or an indy/kart car, or even just you're own made-up shape.
You may have seen this other thread where I made an open-wheeled M18 with a buggy body
http://yourmicro.com/forums/showthread.php?t=18534
but I'm sure something that is really made for task could look a lot cooler.