We are fortunate to have 3 outbuildings. One is a Tool shed with the Generator and Gas in it. Another is totally full of prepper stuff. The third is full of my daughter's stuff. Then we have a 1/3 of an acre of Woods. If something was to happen to the house, we'd be pretty screwed because the really good preps are stored in the house. I've been waiting for the price of those 20 foot metal shipping containers to get cheap. Lately, I've started to see more and more signs along the roads advertising cheap shipping containers. The reason I want a shipping container is that I can use it as a living space if necessary. Before you say anything, I don't want a 40 foot Shipping container.
I sell the containers, they will not be getting cheaper anytime soon. If you want one, get one - if you want to wait for whatever you think is a good price than you can search FB, craigslist, or other online sales sites.
As far as turning one into a living space, its done all the time but the containers need to be properly set up with ventilation, plumbing, electric and ventilation. I mentioned ventilation twice because its that important.
FWIW, Up here we are selling the 20's for $7k+ and in the summer months they were moving all the time, now its closer to winter and not as many sales, its an Alaska frozen ground issue and sales always drop in the winter