Oops, I meant 36 or 48 spoke wheel. A regular 32 spoke wheel will be the week spot on a bike like this and you will probably break spokes if you carry huge loads.I have a 203mm rear disk installed and it works and it looks cool.
I really like big tires on cargo bikes so I would make sure my rear wheel was at least a little wide. Rhino lite rims would be about the narrowest I would use. There is tons of clearance in the back for tires, I think I can even fit a Surly Endomorph tire!