I just winged it after looking at the heavy duty commercial models available. As for what I'd do different, I'd use wheels more like what you made up rather than the rubber ones on mine. Something that I will probably change out "someday" probably after one of the rubber ones fail completely.
I would probably make the firebox a little smaller I think I lose a good bit of efficiency with it the way it is.
Another thought for you, when I built mine I inadvertently added a bit of slope to the whole thing away from the fire box. (tactical error adding the wheels
) but it worked out, the grease and drippings tended to collect in the bottom away from the fire box so I just drilled a hole to allow the grease to drain out and set a pan under it to collect it.
The paint I used was what ever name brand high-temp barbeque/stove paint the farm store had on the shelf, it needs a touch up but hasn't been repainted in almost 16 years of use and weather.