Found this link which tells you how to determine if your machine is a high or low shank. It also tells you how to modify the big foot for more visibility but it works so well now that I don't think I'd do anything to it. The price the have for their big foot is about the same I've seen other places...I suggest that you also check with a Singer dealer to take a look at the darning/free motion foot they have so you can compare the two styles.
http://www.daystyledesigns.com/bigfoot.htm