[ThaddeusMaximus]

Keep in mind: Can you stand to have the output gear that low to the ground? If you're running a 12 tooth cim to a 72 gear direct to a 4" wheel, I would hate to run that because you'd destroy/damage the gear upon going over some of the defenses. For clearance reasons I'd go with a 2 stage. The packaging isn't that much larger if you do it right.

As far as tooth strength goes, the larger the output gear, with equivalent torque, the less force on the teeth. τ = r x F => F=τ/r