Must have, Nitrate, Nitrite, Ammonia kits. With corals you also need Calcium and Alk. test kits. Buy quality kits, most use Salifert.
Also you should buy a refractometer. They are much more accurate than cheap swing type hydrometers.
Also it would be nice to have a PH monitor. If not that get a PH test kit. Thats a good start. That will run you a few bucks. Order them from one of the good on line suppliers rather than your LFS to save a few $$.
You didn't mention lighting or skimmer so I suppose your covered with those items. Good luck!