Firstly, I wouldn't put a coral beauty in anything less than a 55g aquarium. 20 is far too small for it sadly.
The next purchase I would make is some live rock. If you are for some reason determined to keep your angel you will need to buy cured live rock which costs quite a bit more money than uncured and way more than dry rock. The live rock will be your primary method if filtration for the tank.
If you decide to return the angel (I hope you do) then you can save yourself quite a bit of money and buy dry rock and cycle it yourself or go with uncured live rock. Either way you will need to cycle the tank before adding anything living into it because it will most likely die from the ammonia / nitrite fluctuations.
I would skip the reef inserts because imho
they are cheap and don't look very good compared to the real thing, you will have to regularly clean algae off of them, and aren't nearly as exciting to watch grow. With the proper amount of flow and lighting you can keep real corals in your tank without too much difficulty.
If you don't return the angel then you will need to monitor the water daily and make large 50% + water changes accordingly to keep the ammonia down. What are your parameters currently at?