I'll second the motion that if it's your first trailer, buy used and after a year or two of using it, you will have a better idea of what will be your ideal trailer. And, by buying used, you shouldn't take to much of a hit when you sell it.
In terms of smaller trailers, I suggest you check out the Winnebago line of trailers. They are better than entry level and they have a wide range of size and weight ratings. We had a 2014 Minnie 2201DS trailer and it was well made and we only had a couple of things that had to be done under warranty. If you want a 4 season trailer, look for used Arctic Fox but be aware that they tend to be heavier due to their construction, but are great for cold weather camping.
I can see why you would want a Casita or Oliver trailer. But after having some experience, you'll be in a better position to know what size will work for you and what amenities are important to you.
If you plan to camp without hookups, you will want to compare tank capacities and the ability to easily increase the number of batteries and/or solar panels for keeping them charged.
Good luck in your search!
