The price is a little
steep for a 39 y/o trailer, but as long as the frame and floor supports are in good shape, the price tag shouldn't put you off.
The big thing you need to look for is rust on the undercarriage, because you don't want the floor falling out of the trailer. It has been known to happen on trailers that aren't regularly inspected and kept up.
Hopefully the bearings have been greased and repacked regularly, and make sure the emergency brake, lights, and wiring aren't showing any major signs of wear. Also, the crank should be in good shape. I had to replace mine on the Lear within the first 2 years because it wore out. Nothing like trying to unhitch the trailer from the truck, and the crank is stripped.
Check the tires, too. Trailer tires tend to dry rot more than get puncture flats. If they're dry rotted, they can blow out while hauling, even if empty.
As far as the size, it should haul one 15 h horse easily, or several smaller ones. Just make sure your truck has the right towing capacity for full weight.