The newer the better. If you have the time to wait, brand new may be best suited for you. Evaluate how the trailer was used and stored. We bought our Escape
used. At the time, it was four years old, bought from original owner, always stored inside in its own dedicated garage. Paid a high price for it, basically new price. What we saved was shipping to our home, we got several optional upgrades. and we didn't miss out on a camping season waiting for a new one.
Brand new trailers can have issues too.
And regardless of what you buy, how YOU store it when it is not being used is important to its lifespan and condition. My two trailers are stored in dedicated carports. And the smaller one will fit inside the garage. Outside in the elements, sun, rain, and snow, create wear and tear on any camper.
My 1977 Trillium
is a project, I've put a lot of hours into it, and I am not close to done yet, although it is now usable as a camper.