Its a good idea to use ordinary kitchen foil to build up areas such as the chest/torso and hips along with some wire to prevent the maquette from becoming too top heavy when the clay is added as this can cause the model to sag or require more external support than is necessary, too much external support in the form of extra wire can make access more difficult.
If you have turnarounds, and I'm not assuming you will have, then by all means use them to help shape the torso etc.....but don't make it too big as you've still to attach clay on top. Its just for keeping the weight down. you can just about see some foil on the torso in (pic st1.3).

Clay - the only one I've ever used is 'Super Sculpey' which when soft is very much like plasticine but a bit less sticky and when hard or after you've baked it they describe it as a ceramic like sculpturing compound with can be sanded an drilled an shaved etc.