IMO for nitrates, any cheap test will work. It will give you a ballpark number and as your tank matures and your husbandry habits improve that number will go away on its own.
Save your money on a Salifert PH kit. Spend the $90 on a Pinpoin t PH monitor and you will always know what your ph is simply by looking. It can alert you to other poroblems before they become serious.
Buy the calcium, phosphate and dkh. The dkh you will use less and less with time as you learn to watch your ph monitor and behavior of your system, and you will simply know when you need to add buffer.
Mag you dont need to worry about until you start to do a lot of sps and clams. Regular water changes will just keep this level fine by default.
Ammonia is not necessary IMO either. If you wait for your tank to cycle completely ammonia should never be an issue, and if you are missing a fish, you need to get it out. At that time if necessary you can simply take a sample to your LFS to test for ammonia for you.
Salifert prices
here
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