My thoughts would be to look for a used dually, probably 2wd (cheaper and usually has just a bit more towing capacity) with a big gas engine. The reason why is that the diesels in your price range may run well, but the rest of the truck may be worn out. Where as a truck with a big gas engine probably has much lower miles, was more likely to be a personnal truck than commercial, and sellers know that to compete with the diesel offerings, the truck must be pristene (look new, drive like new, new tires, brakes and shocks, etc).
Every truck pulling those loads will drain the fuel tank surprisingly fast! There will be days that you stop and check for a fuel leak. I've recorded as low as 5 mpg in my old 454 (1983) towing around 10K lbs. Of course, the newer trucks will do much better, but don't expect much beyond 10 mpg. Even the diesels will only get marginally better mpgs loaded, so if its not an everyday thing, my opinion is the gas truck will be more economical.
Another thing to steer clear of is the small block trucks with automatic. Not that they are bad or unreliable, but they will not pull as much weight and will be very slow, suck down even more gas, and wear out quicker. For whatever reason, I've seen more transmissions fail behind the small gas engines than big block or diesel engines.
Good luck on your search and post some pics when you get your new ride!
