The only thing to be careful of with the snap-on feet is to make sure that the width of the little rod is correct. I've seen snap-on feet with different widths in that rod.
What kinds of attachments are you wanting? A walking foot is not clamp-on, so you simply need to buy the correct walking foot for your machine. Often it is best to buy the walking foot made specifically for your machine, and they tend to be expensive; however, generic walking feet work well on some machines and are usually more in the neighborhood of $25-35.
Most other feet will not be particularly expensive. I have seen some snap-on feet at Walmart for around $10. Purchased individually, these could add up. Probably the least expensive way to get the feet is to find a set on eBay that you are sure will fit your machine.