Can I store whisky/gin in stainless steel tanks for a month? Do I need a specific grade stainless steel?

Answer 1

Yes you can...and no you don't.
The least expensive stainless is 18-8. It's what you find most commercial stainless cookware and utentsils made from.
304 is good, and 316 is the highest grade commonly available...and also the most expensive grade. Whatever you get of any of these will suffice very well. Just make sure that the tank seals airtight or you will lose some alcohol from evaporation.