Go with something stronger than Polymer clay. It will NOT survive the stress and pressure of being strung, and will shatter. Polymer clay tends to only useful as an 'in-between' medium when going to resin (with a *jointed* doll). Go with either Paper-clay or Apoxie Sculpt. This is a project where you don't want to skimp on materials, if not it will fall apart.
It really isn't cheaper to make your own doll. For what they are, Bobobie and Resinsouls are an absolute steal... all resin dolls, regardless of company, are hand cast, drilled and ultimately sanded to give a nice finish before being assembled. Each and every piece of that doll is hand-made. I'd agree that $200 is a bit much for a mass produced doll of the non-limited variety; but due to it's nature, resin dolls are nigh-impossible to produce in a factory and are therefore not mass-produced. The ones that come with painted faces, those faces are also hand-painted.
Making a doll takes a LOT of time. From initial conception to final form, Allison took 3-4 years to complete.
You might want to listen to Zirc's suggestion on the wooden beads and copper tubes for the internal structure/jointing. This is the technique her husband used for the doll he made.