Add an Alkalinty test kit to your pile. Alk is the balancing factor for calcium, easy way to look at it is that is what keeps the calcium suspended in the water column, too low and calcium will precipitate from the water (it will litterally look like a snowstorm).

SeaChem seems to be the middle of the road for test kits. they are pretty accurate, though some are not the easiet as they require multiple steps to perform. But they are moderately priced.

If you are not using your own RO/DI water (and maybe if you are) Phosphates and Silcates could be another kit later down the road that may or may not be of some use for you. I believe SeaChem has multi test kits that include these with Iodine/Iodide. These fall into the 'nice to have', and generally only used if you think you may have a problem or starting out if you want to be sure of your water quality as you build your reef.

Any additive you consider dosing your tank with, be sure you can test for it before and after.