IMHO & IME, the routine hobby test kits for ammonia, nitrite, nitrate, KH, and GH are not bad - some better than others, but most are workable. The pH test kits are generally weak. The plant nutrient test kits other than Hach and LaMotte are IMHO either inaccurate or painfully difficult to use well or to read or both. Both of those companies have excellent test kits, but you do pay for that - a selction for the planted tank will cost you a few hundred bucks. Refills are much less expensive, but the initial investment is not small.
Iron test kits are (still IMO & IME) not worth much. They really do not measure well enough to give much help. And the warnings already given in this thread are right on regarding iron overdosing.