You ask some very good and pertinent questions. As others have suggested, it depends greatly on the amount of film developing and printing that you do.

When I started mixing my own chemistry, I worked it backwards from the amount of chemical per litre of use solution. Most everything (individual chemicals) has a good shelf life if kept in sealed containers. It is primarily when use solutions are prepared that oxidation occurs.

If you are planning on Pyrocat HD, as it appears, then I found that I substituted Sodium Carbonate for Potassium Carbonate since I already had it on hand for other uses. I spoke with Sandy King prior to doing this. The only change is I mix it in five times the amount of water for the stock solution. I then change the mix to 1-5-100. The reason is that Sodium Carbonate reaches solution saturation much more rapidly then Potassium Carbonate. It just makes my life easier to eliminate different chemicals where I can.

I understand that Michael Smith is still printing with a large quantity of Amidol that he bought a number of years ago.

I have bought from both Photographers Formulary and Artcraft Chemicals and I prefer the latter for reasons of service and price.

It is my understanding that in regard to TF4 which is a proprietary Photographers Formulary formula the issue is one of a stop bath, not of HCA. Due to the fact that it is alkaline it requires no stop bath since the developer is alkaline and to transfer an alkaline developer into an acid fix is to neutralize the fix and shorten it's useful life.

The issue with HCA and thiosulfate based fixers is that the thiosulfate compound is difficult to wash from fiber based prints and the HCA converts the thiosulfate compound to a more readily removal compound. I believe that I am correct in this. If not I am sure that someone will step up and straighten us both out.