I think for nitrates and stuff api is fine.
When you get to calcium, alk, mag then I would either get salifert or elos.
I prefer the salifert though over elos.
I'd assume that's pretty accurate, but I don't know if it's worth the money. To tell you the truth I never test ammonia, nitrate, nitrite. Haven' since I first started to make sure everything was good. If anything I would just get salifert nitrate kit.
Do not waste your money with the monitor. API for your nitrate should be more than enough. You probably will not need it later on. I have not tested for nitrate in years. For CA and MG then you want to use something else like Elos or Slifert as AKMA said. I prefer Salifert too.
For ALK I use API as well. It is fast and since you want to keep your ALK in a range of 7 to 9 that's enough.