Well, the sun corals thankfully won't need any light. They are non-photosynthetic and need to be fed. Can be tricky.
BUT, everything else are lower light corals. With a 20 gallon tank, you can easily get away with a simple 2 bulb T5HO as long as you aren't wanting to get anything that requires more light. If you do, things get more expensive unless you get yourself one of the cheap chinese LED
panels...but then you can keep anything you'd ever want under one of those.