A few comments.
Your fish are going to get very large for that size tank. They'll probably be OK for a little while, but an emperor and a blue-jaw trigger can easily exceed 6" in a year or less.
For dosing, others have already covered the basics. For the most part, you don't need to do anything now, as the tank has virtually no demand for calcium or alkalinity. You might want to check the alkalinity from time to time, as the nitrate cycle will consume a little; this can be corrected with a baking soda solution made from store-bought baking solution at a concentration of 78g/Liter (buy a kitchen scale and a 1L volumetric flask off of
Amazon - they're about $15). Alternatively, you may simply choose to do 20% water changes per week, which should be enough to prevent your alkalinity from falling too low.
Once you introduce stony corals, you'll need to think about increasing your Ca and Alk testing; most here would advise testing your alkalinity once every 2 days or so until you get a feel for how much your tank's using, and calcium once a week (calcium falls much slower than alk). Magnesium dosing won't be necessary as long as you're changing water on a regular basis.
While you can certainly implement a calcium reactor, most find it to be a bit much for a smaller tank - they just supply far more calcium and alkalinity than the corals can reasonably consume, and a calcium reactor runs best if it's constant. For dosing, getting reliable dosing pumps is highly important. I'd personally choose between Neptune's D.O.S. system or GHL. If you're in the 'states, it's a preference choice, since Neptune's an American company, and GHL has good support. If you're overseas, you may have a preference for GHL if for no other reason than the extra cost of a Neptune system because of import duties.
Making your own 2-part dosing solutions is very easy from sodium bicarbonate/carbonate and calcium chloride. Not only will this save you considerable money over purchasing pre-made solutions from ESV, Tropic Marin or Triton, it also gives you a degree of control over what goes into the solutions. Most of us SPS folks are control freaks.
