I have two PH meters, a cheapo $20 from china and a $45 from my hydro shop.
PH meters need to regular adjustment, usually they will drift.
My cheap one uses a screw driver to adjust, very simple and easy.
My fancy one has no screw is done with buttons and has never seemed reliable after the first week.
you must use them every day or store so tip stays wet.
If they dry out they are done!
PPM Meter for Sure and it is not a big deal, it's electronic and it reads in parts per million.
an EC or Electrical Conductivity meter gets you to the same place with a different scale.
I use PPM, my EC meter reads both EC and PPM.
So to me its a back up PPM meter.
I like redundancy, I always have when it comes to tools.
I like a point able temp gun, got mine at lowes for $8.
I use that to verify actual temps at certain spots.
Great for finding hot spots.
temp and RH Meter.
mines wireless so I can see the temp and RH from downstairs.
I feel it's better to monitor often.