$15k was mentioned in the first post. FYI. Although they might be out of the price range, the ford 6.4 diesels can be a bargain. They were and still are the strongest stock diesel in a pick-up made. They were horrendous on fuel mileage and needed a only a few things to be bullet proof. Basically they needed a coolant filter, egr & dpf delete, and a good tune. The only other real weak spot was they had a bad run of radiators with poor cimps that had a tendancy to leak and the best solution was a new radiator. If you do the first three things, they were really stout and very good engines. The real probelm was they were the first generation of the DPF system which was the root of most of the issues. They were know to hold 600-800 hp in stock configuration rather well! the key to any of these, finding one that's low mileage and hasn't been abused so you have a good platform to begin with.
FYI, there's nothing wrong with the old Vortec 8.1's either, they were thirstier and some had a tendency to burn a little oil, but they were a stout puller and reliable. If I was looking at 2011 and newer rigs, I'd probably lean to a 6.2L ford. Every towing test puts that at the top of the current gen of gas engines in 3/4 ton trucks (up untill this year). The 6.4 Hemi makes more power but cann't get it to the ground due to transmission gearing and the GM 6.0, although reliable, has never been a power house and has pulled the rear spot in comparisons for more than a decade.