First, Steve is right. (My, that hurt!) ;D
Payload, GVWR, etc. determines if the truck can pull the load. If it can't, nothing else matters - WRONG TRUCK!!
From my perspective, there are two questions here. The first is mechanical details.
I strongly suggest a F350 / 3500 over a F250 / 2500. For used, the price is nearly identical, they are the same physical size (SRW) but the 3500 has a 1500# HIGHER GVWR and 1500# higher payload. This computes to 6000# more 5er assuming a 25% pin weight. A dually pushes the numbers even higher.
Diesel is also a no brainer for me. It is a stronger motor, better low end torque, and higher fuel economy.
It must have integrated trailer brake controller. Most newer trucks above the base trim line have this standard.
4 wheel drive is very nice.
Long bed is very nice and will improve handling.
Factory wiring harness in the bed is nice.
Rear air bags (per Steve) are nice, but are an aftermarket add on.
The second consideration is trim line - creature comforts.
Virtually all newer trucks have A/C and power door locks. Many have Cruise.
As you step up the "food chain", you get cloth seats, power windows, brake controller, better radio
The next step usually adds leather seats, heated (sometimes cooled as well) seats, dual climate control, nicer radio, satellite radio
Next step includes sun roof, nav system....
The trim package has little effect on the towing capacity. More "stuff" will add weight, which will decrease towing capacity a little. We each have a level of comfort we demand and are willing to pay for. That is strictly a personal decision, but will clearly affect the price.
IMHO, nav systems are not worth the money. Aftermarket units are easier to update, offer a much wider variety of features and are usually cheaper.
HAVE FUN SHOPPING!!