What ever you get, be sure to check the manufacturers towing specs for the vehicle. DO NOT believe what a sales person will tell you.
The trailer should have a placard showing Gross Vehicle Weight Rating. Be sure your "car" can pull that much. I will guess a single trailer will be about 2000 lbs then add 1000 lbs for horse and another 100 or 200 lbs for tack and feed.
I would suggest no smaller than a V6 engine.
Other factors to think about are: how far will you travel with trailer? How often? Will you be on interstates?
YOu don't want to be the one on the interstate going 45 mph in a 70 mph zone.