by Outback
I have to confess, I don't know what the vehicle in question looks like. So, I can't give very detailed answers to your question. Any pics available?
Step one: Plan everything very carefully. Are there any moving parts? Any removable parts? Is there a base vehicle available, or are you going to have to scratchbuild the whole thing?
If you have CAD or drafting abilities, draw yourself at least a preliminary layout of what you're going to need.
I highly recommend styrene as the material of choice for the body. You may find it necessary to reinforce some of the chassis or suspension with other materials, such as wood, brass, or even steel. But styrene is a very forgiving material, is readily available, finishes well, and is easily formed.
If your subject has compound curves then I definitely recommend styrene, especially if you have access to a vacuum-forming machine or can make one yourself. Nothin' beats being able to make more parts for yourself when something goes wrong!
Hope this helps. With some more information I may be more helpful.