Keep in mind that if you dislike the pump/filter that came with the pool, it's really easy to replace it with something better. Just unhook the hoses. If you skimp on the surface prep, fixing that is impossible. So really concentrate on getting the site right. Dig down to level; building up doesn't usually work. Every year we see posts saying, "My pool is sinking on one side. How do I fix it?" Sadly, fixing means emptying and dismantling. This is one of my favorite threads on the subject of levelling
I need advice on leveling my ground. You can use the search box to find more threads on any subject.
Size? Go bigger. If you follow our methods, your water will be so crystal clear and sparkling, so non-irritating and odorless, that all the neighbors will want to use it. So you'll need room for all your new friends.
We've had many good reports on the intex sand filters and SWG. Go big on both. Bigger filter means less cleaning, bigger SWG means less pump run time to keep the chlorine levels up, which means smaller electric bill.
If you want to go first-class, install a through-wall skimmer and hard plumb the pool. Use the search box. I know there are several threads with photos.
Order a test kit right now, so you'll have it to test the fill water as soon as it's circulating. Hopefully, it will be metal-free.