Thank you all for your thoughtful replies.

I haven't, in an online search, found the well-system tanks mentioned by KR20. Does anyone have a link? if I were to try to find one of these at the hardware store, it'd help to know what they're called and what they look like. I wonder if a city hardware store would have them in any case.

I agree that old water heaters are a gamble. I'd use new for that reason.

Thanks for the advice about draining them. I never remember to.

While looking for water tanks, I did find these nifty blue barrels. The water preservative, whatever that is, is what would make these alright for me. I think I can rotate the stock once every 5 years. Probably. So, to hijack my own thread, what do you think of using these instead?