The wife and I bought a new 21' toy hauler about 22 years ago and we learned some things....
We selected that length because that was the minimum needed to park our two ATV's inside. Once we reach our destination the ATV's come out and the space is quickly converted to living space. It's worked well for us in our rock hounding trips.
Lessons learned are: Keep an eye on the roof because they develop leaks easily and if they do the water will run down and damage things. Keep an eye out for dry rot inside the ceiling, floor and walls. Many trailers aren't properly sealed underneath and road water splashes up and contributes to deterioration.
The tires that come on almost all trailers are trash and if you tow it very much you might want to put on 10-ply truck tires for safety and durability. Most used trailers have tires full of checking and dry rot.
Make sure your truck is rated to tow a trailer of your choice. One of the most ridiculous and dangerous things I often see is some guy towing a 26' tandem axle trailer with a half-ton pickup. Consider buying a hitch mounted load stabilizer for safety unless you're going with a fifth wheel model.
Make sure the fridge works because the computer board in them is often toast. Plumbing lines should be carefully checked. Be aware that almost all plumbing systems parts in trailers are not compatible with residential parts which need to be bought at trailer supply places.
Time is the enemy of trailers because the materials used in their construction is not durable against years of weathering. Plastics get brittle with time and that's a huge issue with trailers. If possible, buy one that is not too old.
That's all I can think of for now.
... to the New Guys from The ChatBox V.2 Crew