What you need to test for:
Salinity - I use a refractometer and calibration fluid to well calibrate it as needed.
Cycling - Ammonia, Nitrite and Nitrate. After cycling, I pretty much only test for nitrate - I use Red Sea Pro but Salifert is good too.
The other big nutrient is phosphate. Alas, only one company makes a test that is sensitive enough to be useful the Hanna ULR phosphate or phosphorous. I use the phosphorous because it is a bit more sensitive.
Also, having a decent thermometer is a good idea. It does not have to be exact. It really does not matter if your tank is 77 degrees or 78 degrees. But it is good to have an idea of the temp and a way to control it if needed.
Finally, there is the big three. Hard corals build their skeletons from calcium carbonate (and most marine invertebrates use it for hard body parts like snails and their shells), Some magnesium gets used in lieu of calcium (but only at 5% of the rate or so I understand). So being able to measure Calcium, Alkalinity and Magnesium is good. If you have a reef dominated with reef building SPS corals, you really need to keep on top of this. I measure ALK daily and I have a doser (I also hand dose to keep things adjusted). I dose Calcium also. I have found that water changes maintain my Mg levels.
Salifert makes decent tests for ALK, Mg and Ca.
Hanna and Red Sea Pro are also good for ALK.
Red Sea Pro is good for Ca.
Red Sea kinda sucks at Mg and I have not tried Hanna.
If you have noticed a trend here, Salifert‘s tests are pretty good except for phosphorous.
By the way, before getting a test check out you tube for videos on the test. Sometimes a test can be great but if it involves a titration to a color change and you cannot see the color change, well it does you no good. Or the test can be good but it involves matching shades of a color that drives you nuts, then that test is not good for you either. And sometimes, the test procedure itself will rub you the wrong way. It can be a great test but if it sets your teeth on edge, maybe you should find another one.