Age makes no difference. It is all about condition. The first places you should look at on a trailer are underneath to inspect suspension, brake wiring, and the condition of the tires. The second is on the roof to see how well the sealants have been maintained.
Check the corners of the interior and all over the floor for any water damage or soft spots.
I have no input for you on the ladder and handrail. If they are useful for you, I guess that would be a buying point.
About towing weights: The most critical factor for 1/2 ton pickups is the Payload rating. A combination of passengers, cargo, and the hitch weight of the trailer should fall below that rating. The hitch weights you posted above are fiction. You can measure for that using a bathroom scale, a fulcrum, and a stout piece of wood. Scaled weight is always most accurate. A fully loaded trailer with battery and propane tanks on the tongue weighing 6000 lbs. will have a typical tongue weight of 660->900 lbs.
