Here are the towing capacities for the 2012 model year F-250's (from fleet.ford.com). Your payload capacity will almost certainly be your limiting factor though. You will need to get that off of your specific trucks "Truck Safety Compliance Certification Label". Whether or not to go over your weight ratings is a personal choice.
When looking at used trucks, make sure you pay attention to what wheels and tires are on it, as they could easily have been changed to something rated for less than the original.
'04 GMC Sierra 2500HD, CCSB, D/A, MaxBrake, B&W Companion, Retrax Pro, AR Power Steps
2013 Rockwood 8289WS w/Diamond pkg
1969 Camaro RS/SS Convertible
Dad's old 1968 Empi Imp (recently recovered from under 21 years worth of boxes/dust)