Chris brought up good points, Re: used. From my now 10 year old trailer, that is about what I have done to it over the years. If you buy used, I would plan to do all those thing.
He also brought up good points Re: new. Most trailers I have seen come bare. Plan on a spare, a spare tire holder. The lug wrench for your truck wont fit the trailer. And you may want that jack... Same he mentioned for the truck; brake controller, proper harness ect.
Think too, about tie points. I chain to the points that came on the trailer. But, I added a bunch of J-hooks for using rope to tie hay, ect.
My trailer had new tires, and brakes on one axle(7klb 2 axle 16'). I have read numerous times over the last 13 years here about places selling new trailers with no brakes, and/or used tires.
I'd make sure the new trailer comes with new tires. And, I would get brakes on both axles.
The other thing to think about, is chains, binders, and maybe a toolbox to store that stuff(tongue mount maybe). And, the hitch and ball...
Things to think about. And all of them make the $$$ add up too
