I would look for one that turns nice and freely and one with a spoked wheel and like she said a reverse lever
Muv - I love your new video on free motion. It works whether you treadle or not - same principles. Here is a link folks - don't miss it - grab a cup of tea! http://www.youtube.com/watch?v=H2tiiZ51J7g
If you get a Singer 15 you will love FM on that!