YOu will get different opinions from people, but for the price API for Alk and Ca, and Elos and Salifert for Mg. for Po4, nitrate I can not comment. Get a meter for PH.
also may consider looking into hanna instruments aquaculture photometers, they're a bit more pricy off the bat, but eliminate the guessing of colormatching
There are a couple good threads that discuss this along with some polls where people voted on their favorite brand of kit. Here's a link to one of the threads with a poll that you may like to check out.