As far as armor, there are alot of choices out there. I used ply wood. If you do that, get treated ply wood that wont warp. The regular ply wood I used would warp. Not an issue unless you traveled through a rain storm to get to the field. You can use corogated plastic. Some use metal and some have been known to make fiberglass bodies. Now that all depends on what you can do and what you can afford. I have limited skills and budget so it was ply wood for me.
Here is a picture of our sister fields tank. May give you some ideals. (Wolf Creek Paintball, in Star NC)