Mark, as you've acknowledged a lot of good advice has been shared. My two cents; wife pulls a light weight 3 horse living quarters trailer (TrailsWest) with a new generation 400HP Ram 2500. Her trailer's normal towing weight is 10 to 12K. I wouldn't want to pull anymore on a regular basis.
Your proposed trailer, loaded with horses and tack and water and all the other items that'd be in the living quarters is going to come in around 20K. That's a lot.
If I were going to pull that with some kind of vintage truck, it'd be a old vintage class 8 truck. Think old Peterbuilt or Mack. In cowboy country it's not at all unusual to see a 18 wheeler truck pulling big horse trailers. Something like
this would really be cool!Good Luck,
RonR