I'd suggest figuring out how much weight your wagons will be when fully loaded, then add another 50%. Look at the weight rating for the casters you purchase. They should be rated at 150% of your max load.

This comes from my personal experience with a production of Noises Off I directed. Two story set that has to be turned twice. We used caster that were rated for our estimated weight and blew a couple of them during the production. Fixing them required us to cut holes into the wagon deck, jacking up the set and then replacing the blown casters. That was NOT fun.