Since you are only working on clay right now, and in the future, maybe plastic and vinyl, you don't really need an industrial strength dremel. A cheapie little $20 or so cordless will work fine for doll work.
My old dremel is a bit unwieldly, so I thought I'd pick up a cheap cordless so I would only have to drag out the big guns when the little one wouldn't do. But so far, the little one is doing everything I can think of for dolls.
This is the particular one that I've been using:
For small and delicate parts, you can't go wrong with a pin vise. For larger pieces, a decent rotary tool isn't a bad idea. Having a hobby knife, a saw and even a decent pair of plastic model clippers will also help.
If you intend to venture into gap filling, getting some plastic cement is a good idea, as well as a kind of two part putty like Greenstuff, Miliput or any cheap two part epoxy putty you are comfortable with. For putties, consider a shaping tool like a silicone shaper to make your blends smoother, but also to sculpt with in case you want to make more extensive modifications.