Filter pressure is a little high which indicates fairly high return head loss so I am estimating flow rate somewhere around 53 GPM. You could get away with the a 36-sqft filter but a larger filter is going to need less cleaning which is always a plus.

As for materials, fiberglass-reinforced polypropylene is strong, stands up pretty well to chemicals and it is cheaper than SS so I think that is why they don't make many SS filters anymore.