1) Start with the canisters. Find what is easy for you to find and purchase and then purchase at least 3 for each person/mask you want to protect. If the mask is part of a bug out or exfil strategy 1 canister may be sufficient but if the mask is part of a bunker down and shelter in place strategy then 3 canisters is the absolute minimum. Some thing that takes NATO canisters is probably the best idea. 2) Make sure all your masks use the same canisters. 3) Typically the difference between a inexpensive mask and an expensive mask is the level of visibility they afford and how obstructed your view is. This really comes down to a simple question: Do you think you will need to fire a rifle? The bottom of the market is the israeli gas mask which can be found for around $30. These do the job but if you put one on, pick up a rifle and try to get a sight picture and you will see what I am talking about. 4) A water canteen kit or Hydration kit is nice to have and comes down to questions of how long do you think you will be in it and how much physical exertion do you think you will do in it?