You should not have any problem with a 5ft brush cutter. I have a TO-20 and use both a 5ft ford brush cutter and a 6ft Land pride finish mower. Of course like anything else, if conditions are bad enough you could overload things.
As for tractor condition, I had no problems mowing in second gear until the head gasket went. When I pulled the head off I found that the valves were so bad that fluid poured into the ports would run out the valves. In addition the #4 cylinder the piston pin keeper on one side had fallen out and there is a groove in the sleeve the diameter of the pin and about half way through the thickness of the sleeve.
I can't wait to see what the little Fergie will do with a fresh engine.