This, the Dremel 543 Cutting and shaping wheel, for most non-metal materials, makes shaping, fin slotting, fillet removal a breeze! Its at it's best though with a flex shaft attachment.
Buy them all! I recently purchased a pair of 8050-N-18 Micro Dremel Tools. (After the Recall) I never had any problem with either of mine anyway. it's one of the easiest, most enjoyable Dremel tools I own.
You'll never have to much Dremel stuff. Dremel Micro Drill Press, Circle jig attachment, Plunge Router, Router Shaper Table, Flex Shaft, Lathe (if you can find one). and every single bit set you can get your hands on. Oh! don't forget to look for resharpened Mill ends, and Cutters (saves Lots of MONEY). Check Harbor Freight for "Sets" with Sanding, Grinding and Disc sanders.
My dremel does not see much use for rocket building. I use the EZ Lock Cut-Off Wheel to cut steel all-thread to length for ebays, and I use the sanding band/drum to change the dimensions of centering rings. For example to make a LOC 2.56/1.14 ring into a 2.56/1.52 ring.
I use the drill press for rail button holes and sanding down bulkheads and centering rings. The router table works for beveling the fins. I have the circle jig as well but haven't used it yet.
I would say a router jig, circle jig for the router jig, and some small carbide end mills. You can not only slot body tubes but also even cut out fiberglass fins using templates. Probably a water trough so you can keep the parts submerged so you don't have dust...
favorite items: diamond cutoff wheel and the milling bit- much better for making holes in carbon fiber than drill bit. Best advice- buy biggest assortment possible that comes packaged with dermal- that way you don't pay premiums to get flex shaft, etc.