IMHO, the best hobby test kits are made by Hach and LaMotte. Due to their accuracy, they are also the most expensive. If you want to go with the best, that's the way to go. You should verify the accuracy of any kit with known solution(s), especially if you go with the cheaper kits. As far as gh/kh goes, it's a simple titration, and most kits will give you decent results. I wouldn't bother spending money on an iron test kit, not worth it.