I really like my seaChem test kits. I use the Reef Success Calcium kit they make...it detects calcium levels accurately despite magnesium levels. Some cheap kits are skewed by magnesium levels (I can't explain why though).
I'd switch from instant ocean as soon as you can. I started with IO, and I only have a few cups left...then I'll be 100% Oceanic brand, which is supposed to have better calcium levels (judging by what people on the forum claim).
I would really only worry about calcium and a KH
buffer for 'supplements'. If you do regular water changes, iodine shouldn't be needed...same with strontium (these are for the pros who have such elaborate setups that they don't really do many water changes, so they need the supps...and because they likely have difficult inverts that have more demands).