I owned a 2005 F350 with the 5.4L V8, and towed with it nearly everyday. I put over 130,000 miles on it in 4.5 years, and NEVER had an issue with it. I towed a 7500lb trailer, a 5400lb trailer, and a 2000lb trailer on a regular basis for my construction company. All I did was keep up with the regular maintance and the truck treated me very well.
Now in all fairness, I did have a 8.5" Lift, 38" tires, 4.88 gears, and a computer program on it. Gas milage towing wasnt anything to rave about (8-10mpg towing) but it got the job done, and did it with out being in the shop like my Dads 06 6.0L Deisel was repeatedly.
I would look for a good as low milage as you could, with a good maintance history. Do at least the 05 trucks, since those 5.4L were a 3valve and were more reliable than the 04 and older trucks.