I used to pull about 25000 lbs (trailer and ex) behind my S/A. It had air brakes, Cummins L10, and 10 spd road ranger. The pulling wasn't the issue, it was the stopping, which was fine in the dry but downright scary in the wet.
I'd say you'd be happy pulling any machine up to around 14000 lbs. Of course it depends on the trailer and how you load. Make sure you have a good brake controller like the Prodigy 2 as they make all the difference in the world.