It depends on how serious you are about towing and how much power you want.I would highly recommend a mid to late 90's F-250 or 350 with a PowerStroke turbo diesel.It will have all the power you want,but if you do want more there is a load of aftermarket parts that let you make 600-700ft/lbs of torque if you want.Plus,you can drive through the desert in summer with the cruise and A/C on and the thing will laugh at hills and not even think about overheating.Not to mention the fuel mileage while towing will be approximately twice what you'd get with a gas engine.A good example of what you might find would be like my dad's 97 F-250 which he is selling for 16K.95 was the first year for the PowerStroke so I'm sure you could find a 96 model near your price range.
My recommendation for a gas engines would be GM's 5.3 or 6liter in the new style trucks.I have a 99GMC with the 280HP 5.3 and it towed my 21ft eliminator fine.If you have a lot of gear or passengers normally,get the 6liter,300hp I think.My truck would sell for like 15K so it's in your area as well.Stay away from earlier to mid 90's GM trucks if you tow a lot because the transmissions are a joke.Ford's V-10 is a good choice as well but they are hard to find.I'm not familiar with Dodge's stuff,but I know they offer a 5.9 gas and diesel and a V-10 in their mid 90's model.Hope I was able to help you.