I bought a used 21 foot AG pool and am very happy with it. Just make sure that everything is in good shape before buying, including the pump and filter and make sure all the parts are there. I dismantled the one I bought and that helped alot to figure out how it's put together. I had some issues with my filter and had to replace some parts, but it still ended up being way cheaper than new. Keep in mind that the pool itself is not the only expense. You'll spend some money on site prep and things like sand, patio blocks or pads. Mine included all the lumber from the guys deck, so I built a deck so that the kids can jump in, I think a deck makes the pool much more enjoyable. Also, a couple of neighbors have the easy set Intex pools and I MUCH prefer my pool to theirs.
I leveled the area myself and it can be a lot of work. I also dug the middle about 6 inches deeper and am happy with how that turned out. Take your time with site prep and make sure it's done right. My pool is level but I am not happy with the bottom. There are lots of spots where I can feel footprints in the sand from installing my liner. Apparently you can install it without going into the pool. I rushed too much because it was getting late. Doesn't really affect the use of the pool at all, just a bit harder to clean and annoys me. Read lots and take the time to do it right.
Then follow the advice on this forum on how to maintain it.