I pressure-washed it today, which did a good job of cleaning up the wheels and removing all the mud. And opening up more holes in the sheet metal.
The wheels are in better shape than I thought, they still have most of their paint, though even with persuasion, I couldn't get one of the hubs to take any grease. The zerk accepts the grease but it just squirts out from the base, maybe the hole is elongated. As I said though the hubs spin nice and smooth.
There is enough heavy pitting and rust-through that I will probably proceed with building a new box out of wood, just need to come up with a design. I'll get the undercarriage blasted and painted (thinking black now) and get some new tires and be back in business. If I make the box out of wood, I could make the sides removable too... gotta get out a pencil and paper and start designing....