You will only need to dose when wc can no longer keep up with consumption rates. So yes you will need test kit's to determine when and how much. Mag, alk, and calcium will be the big 3, as for which brand that is all personal preference. I like salifert as there quick and easy to use with consistent results. You can buy powder supplements and make up your own or buy liquid, another personal preference.
Test kits, supplement you intend to dose, a way to measure your supplement, and your rate of consumption are all that's needed to get started. Also only dose what your testing for.