I used to own a Featherlite. They are very well made trailers. When you go to see it, pull up the floor mats at the corners especially and look for signs of pitting in the floor. Unlike wood, you can't replace an aluminum floor, so that needs to be in good shape.
I don't know about the width being okay. I would measure your horse head to tail and measure the trailer to see how much room he'd have in there.
$5,500 sure sounds like a nice price for a GN to me, but then I live in an overpriced area.
You just have to see your distance...you don't have to like it.