I would think that properly treated water would last years. But, I'd probably change it before then. I don't know if I agree with the other reply you have, about monthly changing. I realize TX is probably as water conscious as CA where I grew up.

Having helped my father with home brewing and wine making, I think the kegs would be great, if heavy. Wine tends to go in glass carboys, which aren't as durable, and I wouldn't use oak barrels. The only downside I can see is the stainless kegs are probably 10lbs without water: that's 300lbs you'd have to move if it came to it. Maybe just buying some Arrowhead 5gal bottles would be a good alternative, or finding aluminum kegs?