I would not buy a Singer in that price range ($300). The low-end Singers have a terrible reputation compared to, say, low-end Brother machines. I would look for a Brother in that price range, or a used machine from a dealership.
I see no reason why a cast iron head would be desirable. The things most likely to break in a low-end machine are the gears. Vintage machines had all-metal gears but all more recent machines have at least some non-metal gears. The better machines have higher quality non-metal gears that last longer.