Buy dies on sale.
Most useful to me are:
1) strips -any size you use.
2) 1/2 and 1/4 square triangles
3) squares
I don't do applique because of stress to hands and arms and lack of talent or interest, so haven't bought any of them.
I cut strips, sew them good side to good side and then cut triangles from them - much faster than cutting them separately and then sewing them together. I use them for chevron quilts, pinwheels, hour glass, etc.
Strips - so many uses for them, sashing strip quilts, checkerboard quilts. Have fun!