Sweet setup Chin!
Headonkey, a mag 3 would do in a 20L, for filtration I would add on an aquaclear or similar HOB filter with some foam and carbon or phosphate remover or whatever you want. I'd throw a few pieces of live rock and some cheato under the coral rack/shelf.
I'd probably throw a 175 or 250W 14K over a 20L, assuming your growing a mix of sps/lps/softies.
Chris