1. I would go with the 24g, a larger tank is much easier to maintain. Especially for your first tank I would avoid a 12g.
2. Here is a list of essential equipment:
You should get a good test kit for Nitrate, nitrite, ammonia, Calcium, Alkalinity and PH. The last three tests you won't need until after your tank is cycled and you start adding corals.
You also should get a refractometer for testing your SG hydrometers and swing arm hydrometers are notoriously inaccurate.
A good Thermometer not just the stick on strips.
A couple small Powerheads one for mixing saltwater and one for the tank a koralias nano would be a good choice.
Can't remember if the Nano cubes have an integrated heater or not but if not you will need a good reliable heater, the Ehiem Jager and the Visitherm stealth heaters have good track records.
I'm sure I missed a couple of things but that should get you started.
3. Besides this site, The Conscientious Marine Aquarist is a good book to read.
4. Live rock is up to you, but I wouldn't buy much branch rock as it is very dense and isn't as efficient for filtration. I would say 10lbs or so kind of depends on the rock you want some pieces that are very porous but you don't want to fill the tank from wall to wall or you will have issues getting good circulation around the rock.
Optional would be a skimmer they are very beneficial but not necessary.
The lights that come with the deluxe version should be fine for soft corals and many LPS corals.
If it were me and since this is your first aquarium I would not spend alot on a bunch of extras get the essentials and start there. That way you can be sure this is something you are going to enjoy, it takes alot of patience and work to have a successful tank and almost certainly will make you want to pull your hair out at times, but once you achieve a successful tank and everything start to grow the satisfaction is almost indescribable, so congrats and good luck on your first tank.
