Personally, I would try to find a military field ambulance to convert.
Yes, it will likely be a little smaller, but it will be built a LOT tougher, and it will be four wheel drive.
Or, go big and find a military two and a half ton 6X6 (diesel or multi-fuel) with the insulated work-shop body. Then you will start out with a 8X8X16 house to build whatever you want in, and lots of under-the-box room for tanks, storage compartments, etc.. You might even get lucky and find one of the expandable units, which would give you 16X16X8 living area when expanded. But, they are difficult to find.
When you settle on a unit, and start the build, be sure to keep us advised of your progress, and post pictures.
Good luck.
