Research, research, research!
If the trailers are still being made, check out the mfgrs websites to get an idea of the floor plans. If possible, view trailers you're interested in by seeing them in person. Either ask on this forum if anyone near you has such-and-such brand. Or contact the mfgr to see if they have names of folks willing to show you their trailer. (And if you purchase new, provide that name as the company might have a referral program going and that person will get a check mailed to them for a certain sum of money.)
This forum covers all brands of fiberglass trailers. If you're interested in one particular brand, there might be forums just for that brand. Either way, read all that you can so you can educate yourself on what you want.
Decide what are definite needs/wants. For me, it was a full length bed (I'm tall) and a bathroom. Others might and do have different needs/wants. If you really want a bathroom, for example, you're not going to be happy getting a trailer without one after the two dozen times going out at night to the campground facilities.
How many people are going to be using the trailer? Is there enough room for everyone to sit around without being in each others laps and nerves? If you view trailers, take everyone with you so you can see how you all fit.
Do you want to make the bed each night from the dinette area or do you want a permanent bed? Some trailers you have to make the bed from the dinette, which could get old real fast for some folks. And, I dunno, if I had to do that, I'd probably want a foam topper as it doesn't look too comfortable sleeping with those cushion sections under me (I'm a Princess and the Pea sort of sleeper).
Have you ever used a trailer or motorhome or tent before? I never had, so before we got super serious about getting a trailer, I rented a motorhome and took it on the road for over 3 weeks. I knew ahead of time I didn't want a motorhome, but my FJ wasn't set up for towing. I wanted to see if I liked being on the road in an RV and all that. It wasn't cheap to rent, but it was money well spent, as opposed to buying something and then deciding RVing was not what I wanted. After that trip I heard about fiberglass trailers and really started to research them. While I was using the motorhome I got to see lots of other rigs and kind of figured out what I didn't like. And we hit a lot of dealers and a few RV shows. Very few fiberglass trailers are sold
through a dealer network.
Anyway, have fun researching your needs and wants and what will fit them.