First, you need to decide what kind of work you're willing to put into the trailer. Do you have someone who is handy with metal working and repairs? A steel trailer that needs work on the floor, roof, frame, and rust might be a bargain for you. Steel trailers are always going to come with rust and continue to rust unless you take steps to care for them. Aluminum trailers have a rust proof body but can be more expensive, also don't forget that the frame and axles are still steel so you will need to look at the undercarriage of even aluminum trailers for wear.
When buying used you need to find out when was the last time the trailer was moved. This gooseneck might be a fantastic deal, but when was the last time those tires saw road? Do you need to change all four tires before you even hitch it up? Has the wiring been eaten by mice?
Of course, having a dressing room is always a huge bonus. It does add weight and length to the trailer which is something to consider for first time haulers or people with smaller trucks.
So when it's all said and done you have about a million different trailer options and just as many people with their own opinions about what YOU need. Time to make some decisions!