ok, made it here. Do you have a budget? I would guess a stock mtn wheel with a 7spd freewheel and that load has already bent the axle so you're looking at a new wheel. I see two routes out of this. The one mentioned above, a custom rim with heaviest box rim available which can cover a big price range according to the hub used. Or a cheap steel wheel with 11g spokes and another freewheel. I don't think stock machine made 26" wheels are worth the money given the heavy loads you're working with even with constant attention and care as you've got up to 250lbs static weight sitting on the wheel which turns into something else on a descent and skidding turn.

Call up the custom builders and tell them exactly what you're doing and if you can't afford $350+ rear wheel ask them what's the cheapest option possible. You wouldn't be the first person with a budget looking for a solution.