Welcome Benjamin
I'm not an expert, but I've had my 20 gallon nano going for over 8 years and through 4 moves. (I've had other nano tanks simultaneously too).
A couple of pointers i have learned:
You can't have too big of a clean up crew. You will need things for stirring the sand bed, picking at algae on the rocks, and especially eating the extra food (that the fish don't eat) before it turns into fertilizer for algae. A mix of reef-safe hermit crabs, lots of snails, and a few other things.
No matter what... ALWAYS closely inspect coral frags for pests BEFORE buying them. If you don't know about the common reef pests, time to start reading. Most things you can't get rid of once you get it. Especially asterina starfish (they eat zooanthid corals), but including red bugs, flatworms, bubble algae, even mantis shrimp. You can't be overly cautious, so don't be tempted to buy that sketchy looking, "cheap frag" from a swap or a store, cause it could end up costing you a lot more later.
Find and stick to a regiment for adding new things to the tank. More than just acclimation, a "dip" is also helpful to prevent introducing pests, algae, and other unwanted things into a tank.
Quarantine tanks are excellent if you can afford the space (even temporarily).
NEVER ever use a copper treatment in a reef tank. Deadly for inverts and coral.
Don't expect the clownfish to reach maturity in a small tank. Probably won't grow over 1.5" and won't spawn. It's best to buy clowns that have already paired up so they don't fight. And they don't need an anemone, they will rub on just about anything.
Avoid green star polyps (GSP) and mushrooms then. Once you put either in the tank, they will eventually take over. Mushrooms can also emit chemical signals that will prevent neighbor corals from growing out.
Zoas are a good starter coral. They actually prefer a little bit dirtier water than some of the difficult corals. They also can tolerate a lot of different lighting, and look great with just the blue LEDs on at night. Start with the cheap ones.
Eventually you can read their behavior and see what flow and light makes them happiest. Some exotic zoas do have neurotoxins (Palytoxin), so be careful when handling.
Frogspawn and other LPS coral (duncan, Acans, etc) are a bit more finicky and need calcium supplements to grow their skeleton. A lot in this family also have sweeper tentacles (to sting their neighbors) and may not be good for a small tank.
With your current light fixture, you should concentrate on corals that don't have a high light requirement.
With regard to equipment, you don't have to go with a sump setup. My 20 gallon nano has all hang-on-back accessories. I have a CPR Bak-Pak 2 protein skimmer (no bio-bale) and I made a custom pipe to output to a Aqua Clear 110 filter that I use as a combination chemical filtration (media bag with carbon/phosguard) and a fuge to grow chaetomorpha (macro algae that provides nutrient export), with a LED fixture on top.
There are other HOB style equipment available depending on what you want to accomplish (and how much space you have to hide it). Various manufactures of canister filters and media reactors too.
Yep, mushrooms are bulletproof, which also means you can't kill them when they take over ;-)
Anyway, welcome to the world of saltwater.
Read a lot, ask lots of questions, and have fun!
