Honestly it sounds like you have it all under control. I have a planted tank with pea gravel from home depot. $5 for 50 pound bags. All I do for my plants is Easy Green and a cheap light. I've heard good things about Nicrew lights. Good choice. Cory's and Kuhli's enjoy sand. If that's the rout you go I like play sand (also super cheap) and it has a great look to it and is plenty soft enough for the fish. The only thing I am unsure of is how well plants will grow in play sand. I think the plants will do well in the sand because I can see biofilm growing almost 2 inches deep in the sand. I have just never tried rooting a plant in sand.
Once you have your 20 gallon tank set up just remember to only make small changes and only make one change at a time so you can really learn what each change causes in your tank.
Like I said up top. It really sounds like you got this! Keep it up.