I don't know why steel bodied cars last for years, but horse trailer sheeting starts rusting right after you drive it off the lot. That trailer you saw in CT was steel framed, just like I said, rusted. That trailer has a lot of rust. On the upper visible sides, I guarantee the frame and floor supports, are much worse. Steel trailers rust from bottom up. You will spend some money doing alot of welding, grinding, metal replacement, paint, undercoating, new wood, and when all is said and done you will have a used steel trailer that's going to rust. May be worth it for someone who has the time, tools, and ability to do the repairs.
But my hobby is riding horses on the weekend, not rebuilding old junk.