Do some serious reading in the towing section, especially the Can I tow this with my truck" type posts. Once you understand the considerations in matching a truck to a trailer (Payload, Hitch wieght, Weight Distribution and sway control systems, etc) Then go pick out a trailer you like. Then find a truck that will give you a nice safety margin of a couple thousand lbs in towing and 400-500 of payload is my suggestion.
If your going to use the truck mainly for towing, consider a rear gear with more grunt, a 3.73 or 4.10, depending on weight of the trailer. Will give you more "get it moving/merging power" and not work the engine to death on hills.
