I love polka dots! I've collected them for years in all different sizes and color combinations. I particularly love tone on tone dots.
I also save scraps and cut them into different sizes. My favorites are 2.5" and 1 .5" squares and 4.5 x 1.5" strips. I call the strips "fiddlestix" and use them in tandem with the squares. I just made a wall quilt and didn't have to cut one piece, except for the backing, (I stitched the strips together to make the binding).
I also tear off a piece of every fabric I buy and cut a few of each of these pieces. It's fun to be able to grab a box and have pre-cut and pressed pieces to use.
As for other sewing supplies, thread in multiple colors is a must. I recently bought a box full of thread from a thrift store, which is a great place to jump start a collection. Years ago I bought one of the Gutermann quilting thread wall racks. Best purchase ever!
I'd also recommend a great pair of scissors, sewing machine and hand sewing needles, as well as needle threaders, clippers, flat head quilting pins, a comfortable thimble, a telescoping magnet wand for stray pins and needles, marking pens or pencils, a good quality seam ripper, along with extra bobbins and a SideWinder to fill them.
I'd also suggest a fishing tackle or other kind of organizing box to put everything into so you can find it when you need it.