The load our OP is intending to tow is well within the range of the truck he is considering. As most of us know, I have been a Ford man all my driving life, but recently bought a 2500 GMC. I have, like Chris, had some experience with the 150s of the vintage discussed. They tow fine and can handle a 6500 pound load without difficulty. The Ford 5.4L engine is simply not a very good towing powerplant. Unloaded it does well, but towing it lacks the power to pull a pretty heavy 1/2 ton truck and the load without dipping deeply into its reserves and using a lot of fuel. The reality here is that most of us, and perhaps our OP too do not tow that much of our actual driving time. I own a diesel and feel like I tow a lot, but really it is probably no more than 10% of my actual on road time. If our OP is also of this type percentage--or less--he will be fine with this truck. It is not the best out there but is a solid and safe towing machine.
With regard to 2500 level trucks and their offroading prowess, I have found my GMC to be the best truck I have owned in this regard. Generally the heavy suspension and less articulation and the added chassis weight make the 2500 level trucks less good off road than a 1500. Obviously, the best towing vehicle would be anything with a diesel but for our OP it seems a 150 such as his example would fit his needs pretty well, unless he literally was towing all the time. It will struggle on the hills, get pretty poor mileage when towing, but like most Fords will be safe and stable from the chassis perspective. I agree with Chris the Hemi, though powerful for a gas engine, does not tow very well because its powerband and tranny gearing is not well matched to towing duties, unless someone tows at 4000 rpm all day. Those I know that like it for towing either don't really tow much or aren't realy towing much weight.
John M