not to belabor the point, but if you are going to have anything precise, your alk and Ca test kits are probably the two most important. followed by your hydrometer/refracto and pH measurer. (although when alk and Ca are square and you have good gas exchange, the pH takes care of itself which is why pH is #4)
those 4 things, and the relatively little money those cost compared to the value of the stuff in the reef should not be an issue, especially a couple of $15 test kits.