The unglamorous foundation of bulk dry storage. Rice, flour, oats, pulses and sugar in quantity all end up in buckets, because nothing else is as cheap per litre, as stackable, or as indifferent to being kicked.
Food grade is not a marketing word. Buckets sold for other purposes may use plastics or pigments not intended for food contact, and a bucket that once held something else is not made safe by washing. Buy them as food containers.
Standard sizes matter more than they should, because gamma seal lids are made to fit the common bucket diameter. Buy the two together and check they are the same standard, or you will own a lid that fits nothing.
A bucket is a light and pest barrier, not a vapour barrier in the way Mylar is. For a year or more, the usual arrangement is a sealed Mylar bag inside a bucket, which gives you the barrier from one and the stackable, rodent-proof shell from the other.
Twenty-five litres is the common size and is heavier than people expect when full of rice. If it has to be carried rather than slid, buy smaller.