I've done a little towing - you want a full size truck with the longest wheelbase you can find - this prevents the trailer wagging the truck syndrome mentioned before.

You are probably best off with a 1 ton suspension setup, since these often have beefier rear ends, which will last longer.

Old trucks are fine and often cheap, but they may require a lot of mechanical maintenance - don't know who would work on your truck ...

Good Luck - you don't need anything fancy, just a long wheelbase and a 1 ton suspension. Diesel will get better gas milage and the engine can be rebuilt forever but it will be probably be geared lower.