This is our third year with an 18' round Intex easy set, and it's been great. We use all the original equipment, although we do use the TF good test kit. A couple pieces of advice...
A lot of people on this forum complain that the filter/pump are inadequate for these pools. I'll definitely agree that they're nothing fancy, and Intex sure didn't give you more than you need, but in all honesty, we've not had a bit of trouble keeping our water clear with the equipment that came in the box. And although Intex says the cartridges are meant to last only a couple of weeks, we've found that cleaning the cartridge every week to two weeks with the garden hose, and maintaining adequate FC levels, we've been able to extend the life of the cartridge filters without sacrificing water quality. I hose off the filter once every couple of weeks, vacuum as needed with the equipment in the box, and it's been fine. Now, that said, I will say that I'm pretty strict about the two-three minutes a day to test water and add chlorine. But, we've had crystal clear water AND great test results everyday each summer we've had it. I only shock as test results indicate it's needed, and once in awhile after a big storm.
BBB has worked fine for us, although I do occasionally use pucks when we're gone for a few days. I assume you'll be taking down the whole pool after swim season, but still go ahead and get CYA in your pool so you aren't using so much chlorine.
As far as calculating gallons...another poster mentioned that the sides slope inward as they rise, so the pool calculator doesn't work. Take an average of your bottom diameter and top diameter, and use that for an approximation. My pool is 18' at the bottom and 14' at the top (found the top diameter somewhere in the intex info), so I use 16' as my average diameter. It's not so much different than when people have to estimate gallons for irregularly-shaped pools.
Good luck!
Edited to clarify that Intex cartridge filters are meant to be disposable.