The best deal is to buy the color changing resin in bulk as it comes out much cheaper that way. You get many refills in one 7.5lb bag of resin, I think 6-7 in each bag if i remember right. And with the color changing resin there is no guessing when it's spent.
It costs a little more up front but you save a lot in the long run, Plus you can split one with a friend if it's too much for you at one time, just make sure it stays sealed and moist or it's no good. Sediment filters are pretty much standard are OK and Carbon you can get the standard or go for better ones if you have really high TDS incoming water or other contaminates in your water supply.
Bill