I read through this thread and must say that I'm always amazed with how willing folks here are to help a newcomer. You're in good hands here, so welcome. I've also been treated with excellent personal advice so in an attempt to pay my good fortune forward I'll offer some thoughts of my own on your behalf.
Some years back I rebuilt an Amerigo
RG16. It was a lot of work but I enjoyed it because I had the space, tools and time to do it. I very much liked the structure of it and felt that it was a pretty solid piece of manufacturing. Having owned other fiberglass trailers as well I felt that it held it's own in may ways. For what it's worth I include a couple of pictures of the finished trailer.
We all travel a bit differently, have different needs, expectations and feelings about what would constitute the ideal solution for our purposes - all of which can change depending on any number of variables.
Truth be known a small FG trailer like an Amerigo
is very easy to heat because it's such a small space anyway.
Comparing what most RV owners consider a three-season vs a four-season trailer, well, in actuality there is not a whole lot of difference between them if the criteria for comparison is based on insulation alone. Most trailer owners who occasionally camp in cold, winter conditions don't even try to use their plumbing the same way they do in summer. They've likely winterized it and opt to restrict their water use to portable water containers they can access from the inside of the trailer.
As well, if a trailer has a built-in bathroom, occasional winter campers often use a portable potty they can easily empty most anywhere without having to hunt down a dump station. Often times that task ends up happening at home.
As for heating, there are many discussions on this site that talk about alternative ways to warm the interior of a small FG trailer. As well, where you camp will have an impact on which solution will best serve you. A high humidity area will promote condensation in a lightly insulated trailer. (Which can be variably offset by using a dehumidifier.) The temperature differential between outside and inside will dictate how much condensation you'd experience. Speaking for myself, a little condensation and nippy air never stopped me from enjoying a little off-season camping - especially since finding a great place to camp is so much easier in the shoulder weeks/months on either side of "camping season."
Without letting this get too long I'll leave you with this thought: Carefully consider what your realistic needs really are and choose a trailer that best fits your style of camping and go for it. You can always sell a trailer if it turns out to be woefully inadequate. There will always be compromises to have to live with no matter what, anyway.