Horseluvinguy makes a good point about age of trailer vs age (must say I didn't twig on to that when I first read the post). Where I am, a trailer of that age would probably be around $3k (regular trailer not a Brenderup). Perhaps, though the Brenderup is in mint condition and has been exceptionally well maintained???
Is it possible in your area to get a smaller bumper pull (probably 12 ft; maybe 14 ft) stock type trailer. They won't have many amenities but that will mean the price and weight are as low as possible for that type. What you would get out of it is a kind of a slant trailer and the newer ones have sufficient height to them for a horse (6'9'' or 7'0''). I have seen that style, brand new, sell for $7,500 here (just as an fyi). So a newer used one, if you could find one, would definitely fall within your budget.