I think I'd rather just buy a used utility truck with equipment body. They aren't that expensive and have the advantage of being mobile on and off the road.
But if you want to stick with a home built, start with a trailer kit like the 4x8 box store models and build up from there. Then you can tow it on the road as well. If you're going to be in the field or off pavement, then big road tires will be your friend.