Relative to shelf life, I have wandered away from mixing my own and come back several times, so I have used both metol and sodium sulfite that are pushing 30 years old, no problem. With water, I *believe* consistency is more important than purity. . .at least that has been my experience when moving from place to place. I have always used the local tap water.

I might mention that I I use D23 for xray diluted 1:7, and it does not go bad in a sealed tank, however it doesn't do a lot of sheets at that dilution so I usually mix fresh for that reason. You can buy sulfite in 20 pound lots dirt cheap.