I agree with holding off and buying only as you need them. The dies that have proved most useful to me are the triangle dies (because all the dog ears are cut off, making them very easy to piece together) and dies with unusual shapes -- such as the tumbler dies (easy, fun, small great for baby quilts), hexagons, circles, etc. I also like some of the pattern dies, such as the hunter's star, that make cutting and matching pieces for these blocks very easy. I am not a fan of the strip dies because I actually find it easier to cut my own strips using my June Tailor mat; however, many people like them.
If I were ever to get into making fusible applique quilts, then I'm sure the applique dies would be lots of fun.