Um Your in the wrong hobby
None of them are really necessary, unless your a gear junky then half the fun is calibrating them.
If you have a CA reactor I would say get the pinpoint PH Monitor. Even if you don't I think its the most important. But it also requires the most maintenance.
A refractometer is cool but not as important. I would say it's the least for maintenance as you can calibrate it with RO/DI water.
Jason