I buy dies anytime they are on sale. The more dies I have, the more I like my die cutter. Since you are just getting started with it, pick a quilt you want to make, and get the dies you need for that quilt.
When I started out, I didn't think I wanted what I think of as "piecing" dies. I started with just the hexagon die. But now that I have all of the HSTs and QSTs and triangle in a square and so forth, I love when I can cut out my whole quilt with the dies I already have. I much prefer my individual dies to my BOBs, but that is why they offer both styles, so we can each have what we prefer.