once your tank has cycled, and you do your water changes and have sufficient filtration, you need never test ammonia, nitrite again. All I test is calcium, alkalinity, pH, nitrate, phosphate, SG, and kH. All my readings are normal every time. Monitors are quite expensive, require calibration, and a stable system will not fluctuate much. Monitors are chemically specific. Each monitor is only useful for one test.
I'd save my money for livestock. Just a thought. If I was to get a monitor, I'd only spring for pH. And a thermometer of course. Heaters die without warning.
Hope that helps.