IMO - (having done lots of towing miles with my 4R) - Firestone Air Springs for the rear and a Curt Echo. Air bags install easily in an hour or two. No installation necessary for the Echo to control the trailer brakes.
That and a small air compressor is all you need.
A trans cooler could help, but is not necessary for a small trailer like that.
Good luck.
Handled my bigger trailer well. Only issue I really had was with power/gearing on the passes. I also had an F250, but usually still towed with the 4Runner because it's just so much easier to live with in the parks and around the places I want to go explore when we're camping. The 4R isn't a great tow platform, but it's perfectly well suited for a smaller airstream like your aunt has.