I have used the Red Sea kits, they are reasonably accurate. The Marine Lab is good to start with, tests for pH, Alk, Ammonia Nitrite, Nitrate. You need to have these.
Salifert tests are among the better ones but they don't make a 'master test kit' you have to buy individual tests.
Seachem is what I use - I've compared Red Sea results to Seachem and they are consistent.
I don't use or recommend Hagen tests - had shoddy results with them, so I quit using them, and have never sold them.
You can get extra kits for, Strontium, Magnesium etc., but IMO they really aren't necessary.
I would get a Seachem Calcium test if you're keeping a reef. They are pricey but you get a LOT of tests and the tests are accurate.
Seachem tests come with a reference solution, so if you are in doubt about your results, you can test the reference solution to calibrate the test.
I'd keep the Red Sea test and use it, but when you need more tests, I'd consider switching to Seachem or Salifert.
HTH
Jenn