You are going to buy used. Probably 26 feet on down. Look for general condition. Tires should not be over 5 years old or you are going to have to replace them. Make the seller demonstrate all the appliances -- fridge, airconditioning, water heater, furnace, water pump, and toilet. Be sure that there are no soft spots in the floor. Eyeball the trailer to see that it sits roughly level side to side. If there are slide outs, make the seller operate them.
Tires should not be over 5 years old or you are going to have to replace them. In fact you can almost assume that will happen. Same with batteries.
Be sure that your tow vehicle can pull the maximum vehicle weight rating of the trailer. Tow ratings are available for the past 5 years of vehicles in the Trailer Life website. Allow a 10-20% safety factor in those ratings. In fact, you should know that rating before you start looking at trailers.
Just my two cent...
When you found the trailer you like, try to take somebody, who is more experienced than you, to have a look at it before buying.
Never bye the first time/day you see the trailer.
Well, thats how I would do it.