An Aquaclear 30 or 50 would be perfect.
As far as substrate, since you're keeping limited plants, just getting a bag of Amazonia would be a-okay for your purposes. It's great stuff.
On my 20Ls, I use two different fixtures (depending upon the tank and its location). The 30" Coralife T5NO fixture is awesome for low-medium plants and is about $40 at most. Marineland's Double Bright LED fixture is also a nice choice but is easily $70-$80. I'd go T5NO all the way for sure.