NewBee,
I'd suggest two things.
First, find an online towing capacity calculator spreadsheet. You'll need numbers from the sticker on the door jamb of your tow vehicle and from your owners' manual. The various limits will be specific to your vehicle, including its engine, transmission & tow package. When we did this before buying a trailer, we had to make some guesses & assumptions about what the trailer would weigh. Some number will be your limiting factor - likely tongue weight or gross combined vehicle weight (truck + trailer total). Use 15% of the trailer weight as the tongue weight in your estimates. All this will give you a maximum weight your trailer can weigh, including all options and everything you put in in (plan on 500-1000 lbs worth of 'stuff').
Second - right now is RV show season. Go to the nearest show. Take a camera and notepad and look at floor plans and layouts. Look for the basics, don't worry about options & shiny flashy details. Do you like hybrids (beds slide out)? Bunk beds? Bathroom in the middle of the trailer or at the end? Do you need a bed you can walk around or is climbing in ok? Look at the ones in the range of what you can tow. Look on the trailer tongue or near the door for the sticker that tells you the gross vehicle weight of the trailer. This will give you some ideas as you look at used trailers.
Good luck in your search.