I agree with sporto0. It is possible to keep and SPS tank that small, but it takes more equipment and experience to avoid the water parameter fluctuations that can happen in such a small water volume, and that you don't know what's required (and are using a HOB filter

) for such a setup just indicates that you aren't quite there yet.
I'd raise that MH up a bit, you might find it a bit intense for a 20 gallon, but then again maybe not. I'd also stick with easier corals (zoanthids, soft corals, and LPS) for a while until you get some more experience.
As far as what else you will need to keep SPS for the long haul:
ATO system
Calcium/alkalinity dosing system (kalk, 2 part, or calcium reactor)
Test kits - Ca, Alk, Mg, PO4, and refractometer at minimum
Carbpn/GFO reactor
Appropriate return pump and plumbing (you didn't give any details in this regard

)
Heaters
Tank controller (to prevent disasters like an on position heater failure and to monitor things like pH)
A way to provide more chaotic flow
And, perhaps most importantly, a mature tank, completely cycled and up and running with fish and less demanding corals for preferably over 6 months.
It seems like a lot, and it is, which is why it's probably better to wait on SPS until you've got some more experience under your belt. You certainly have a good start, and I look forward to seeing the progress of the tank!
Cheers!