I think you are leaning in the right direction. It's always good to have a flash option available, and the 580EXII is a powerful and versatile unit. All the more important as the 5D II lacks a built-in unit. I also agree with you about the value of those prime lenses. I don't think you'll regret expanding your kit in this way at all. You'll have a lot of fun learning how to use flash with this camera.