If you buy jerry cans only get real mod ones. These are galvanised under the paint. The quality of the galvanising varies with the year, some of the 1950-1960 ones are very good. They are date stamped on the side. Look at the base to see whether the paint and galvanising have worn through.
The non-mod ones will rust almost instantly. The paint on the base will wear through by the time you've filled it with fuel, if it's not already scratched through when you buy them.
Craddock's ones vary, I've had good ones but also rubbish ones. Problem is, unless you go there, you can't see them first.
Is your fuel tank above or below deck?