Ok, so this one provides potassium and Fe.
As far as tests, you probably already have a Nitrate test kit -- this is the N. You can pick up a P (PO4) test kit at a good LFS. You can find a K test but it is not necessary, and an accurate test kit at the levels you are looking for is going to be expensive. A cheaper K test kit isn't going to give accurate readings, so your best bet is to just skip it. Just add what the recommended dosage on your kent bottle, or even 20% or so more. It is tough to overdose the K.
Depending on what substrate you have, I would recommend picking up a micro fert of some sort. The first Kent product I referenced would work (if you need to add N to your tank too) or Flourish makes a "comprehensive" that has all the micros but Fe in it.