I know some things about Marine tanks from my research towards getting one of my own, but like you, lots of money I don't have yet and time lol.
But what are you going to do? Fish Only With Live Rock (FOWLR), only Fish, Reef tank? I assume you'd likely to a FOWLR to begin with, the live rock will help introduce the bacteria you need and help seed your cycle so you can add fish sooner than if you started with dead rock and dead sand.
I do suggest switching out your substrate for live sand which can be bought at the store. That will also help on cycling but also has more calcium/magnesium in it from what I remember which is necessary for harder water (ocean water is around 8.1 IIRC or somewhere around there). If you have soft water, you're going to need to do R/O water and use chemicals to increase pH, hardness, and alkalinity as well. My well water is at 5.0 which clearly, is no good for a Marine tank lol.
Your stocking looks good from what I also remember. And then of course you can do the snails and I think a feather duster too once everything is stable and going well! I love feather dusters lol
I know the API tests will do fine but I also know a lot of people recommend the Red Sea kits as well, not sure what's available to you though, you'll have to look around if not.
And yes, you should find Marine salt at your stores, you should also be able to purchase premade salt water as you won't do huge water changes each week, 5-10% is the normal. You don't want to change too much otherwise you'll through your system out of whack. Do not use Aquarium salt, it's not the same as Marine salt (AQ salt has two components; sodium and chloride, marine salt as over 27).
Do you know about Sump filters? Very useful, especially for if you'll be doing any corals in the future. It's still good to do, even a 20 long will be fantastic for a sump under the tank to keep your tank more stable. Basically, you want as much volume as you can to keep it stable. It's possible to do a regular filter but it won't be as stable is all so you'll have to watch it like a hawk.
Wilds: Wajok, Rutilans, Siamentorialis, Hendra
Leopards: Luna, Astrael, Dorian, Cullen, Gallifrey
African Fat Tails: Nox and Lux
Bearded Dragon: Stormfly