I might consider getting an all-in-one system since it's easier, and you know which things are 'compatible'. I am not sure that an ATO is mandatory (it's fine if you want it) - I like the Tunze Osmolator. With a small tank it's easy to just add RODI to your tank now and then (IMHO).
You will probably want an RODI set up OR - again with a small tank, you can buy RODI water from an LFS. Chemicals - I don't think you necessarily need 'anything' - since you can probably manage things with water changes for that size tank.
As to testing -You can pick your brand (I personally use API due to its price, and in my estimation as easy to use and as accurate as other testers - many will disagree with this though). You will want a refractometer to measure specific gravity.
As to salt - I don't think there is a huge difference - the important thing IMHO is to stick with one brand. You can also get pre-mixed saltwater at most LFS.
Then it comes down to adding water and fish. I would consider having a plan with regards to Quarantine (are you going to do it or not). You will hear 100 opinions about cycling - I personally like
@Lasse article - how to set up a reef tank. here is the link:
https://www.reef2reef.com/ams/15-steps-to-starting-a-saltwater-aquarium-the-lasse-method.597/
Hope this helps! Good luck on your reefing journey