Hi Stacy! I have a cordless Dremel and I love it to death. The attachments that came with it only included one drill bit. I bought a pack of 5 or 6 that pretty much span the sizes you'll need for clay, from very thin on up. You'll also need a set of different sized collets (the little 4-sided metal piece that holds your bit or mandrel in place. You can buy them at Home Depot or any other place that sells Dremels.
I haven't made any muslin wheels yet, but I did buy some at the last gem fair in my town. They came on their own mandrels and for something like $1 each, they were the best purchase. Buffing up already sanded beads with your Dremel will send you into ecstacy (when you aren't chasing them down when they shoot out of your fingers). It's amazing the transformation and glass-like surface you'll get.
Have fun!