Are you going to have frags only or any working fish? I have seen a lot of frag specific tanks contain smaller fish that work like tangs. This of course has its own challenges as they will need to be homed when they grow and all that stuff.
The reason i ask...
If this is to be setup like a show tank for mobility and ease, then i would run a small carbon reactor and filter floss. Check out my buddy at North Texas Corals. I forgot what it is called, but he bought a new reactor that is sexy and fits these tanks great.
If it is going to contain fish, or be more permanent, then i would run a nano skimmer as well. This will help with your nutrient export and give you some breathing room on water changes. If you are growing corals out, you are going to want FSW for some of your changes versus just your DT water to ensure the highest of nutrient replenishment.
The reason i am pushing the carbon reactor is you will most likely have your hands in this system more than you DT, and with its size, you will have lower tolerances for variables like the oil on your skin, etc. Carbon will help offset this.
So short answer:
Carbon reactor, mechanical filtration (sock or floss), small protein skimmer. Start with that and then see if your tests show you need a different or more medium.