U tube and bernina.com both have videos on what each of the feet does and how to use them. I think if you go and watch a couple you will know which ones you will use.
I love the number 57 for piecing. It is a 1/4" foot that has a guard on it to guide the fabric. I also love the stitch regulator for free motion quilting. The built in designs are wonderful and you will enjoy using them on fashions so make sure you have a good embroidery foot. I also bought the flat fell foot and the accessory foot (#71) for the Westalee quilting templets. I think the best way to buy is to get them as they are needed to do a project. That way you are getting what you will use.