We are planning something similar. I've sourced a pair of 275 gallon food grade totes to put at the end of the house up on a platform. We'll run gutter from the front and back of the house and carport (attached) to the totes and then use that water for garden etc. Also going to put a smaller blue 55 gallon drum on the chicken coop once we decide where it's going to be built. Metal roof on both.
The totes we are using are the white ones with the metal cage around them and forklift notches underneath. If you decide to do the same be sure to get the ones that have a green lid and green handle on the outlet valve. Green is good since it indicates the tote contained something safe for human consumption. Do not use the ones with the red or black lids and valve handles since they usually carry cleaning chemicals or caustic substances.
Hope that helps!