I've been learning vacuum bagging from a RC cargo airplane team/research group, where we have a foam wing that we lay uni-directional carbon fiber and bi-directional fiberglass onto to increase strength (the 100" wing has to be able to support 40+ lbs).
If you want to reuse the mold, you'll have to wax it before each time CF is layed down. The wax needs to be rubbed in really well, as any minor chunks will show up on the finished product (like pb4ugo mentioned, the mold surface needs to be smooth).
The usage of a fabric ("batting") is to hold the excess epoxy, so it doesn't get into the vacuum pump.
That website linked to on the first post has thicknesses listed for the carbon fiber fabric:
http://www.uscomposites.com/carbonpage.html
The thickness you use I would think should depend on how many layers you want to do. The thinner the fabric (the lower the weight) the more layers you'll have to have. But I bet if you have too few layers, it won't be strong enough.
Actually designing the mold will be the hardest part IMO. I was thinking clay would work, but then I remembered that clay shrinks as the water in it evaporates. But if you could account for that shrinkage, then I suppose it'd work.
I want to try vacuum forming some CF or fiberglass as well, since I have access to mylar (vacuum bag material), fiberglass cloth, epoxy resin, a vacuum line, and the appropriate wax.