You want to use the "Brute" line of Rubbermaid cans. They are food grade and will not leach anything into the water. I rinse mine down with vinegar before use.
Salt mixes that contain organics, such as Reef Crystals or Red Sea Coral Pro, should be kept circulating until use. Mixes that don't contain organics, like regular instant ocean and Coralife, can be stored indefinitely without circulation. RODI never needs circulation.