Well there is no pat right answer to that question. If it were me and I was looking for a dedicated tow vehicle for pulling what you describe I would be looking for Ford 7.3L or Dodge(Cummins) 5.9L diesel engine in a 3/4 ton 4 door short bed truck with an automatic. These have proven to very reliable and relatively easy to make bullet proof if you have any issues needing major repairs
For reference I am running a '06 Dodge Cummins dually pulling 18k# horse trailer. Have about 140k miles on it with most of it towing. Keep trying to break the transmission so I can drop in an Allison and bullet proof the engine (need a good excuse). I plan to put at least 250k miles on it and if I can't break it soon may have to go to 500k after the repairs (have a cold one bet with a buddy)