Can never have too much truck, especially given the Class 5 rigs are no bigger than duallies and still a pickup body. Only downside with the 01 F-550 is no wide track front axle and it's 8 vs 10 lug iirc, but the rest of the frame is the same as with the new one.
I'd say buy it then change all fluids: diff, automatic, power steering, brake and oil plus filters.
Get the auto checked out but if you can get it for 9 even then it's a decent deal.
Agreed re 3/4 tons, not much payload there.