Thanks for all your advice and opinions guys. I am now leaning towards getting a Hanna test kit for phosphate simply because it sounds like it will give me the most accurate reading compared to other test kits out there. Hanna is a colorimeter and gives you an exact reading, whereas all the other test kits rely on matching to colour charts and depends on the user's interpretation and involves some guesswork too. I will need to also buy the 25 reagent refill because the test kit only comes with 6, which is pretty silly IMO
As for the magnesium test kit, I am leaning towards Salifert only because it's about $10 cheaper than Red Sea and appears just as good. Would still like to hear what others think about my intentions before I go ahead and order them.