Are  you sure you want to use Sculpey for the head?  It has been a long time since I used Sculpey but as I recall it is pretty heavy.  I always used Paper Clay for heads. I would take a styro foam egg in the basic shape I wanted (round or egg) and then cover it in paper mache' of about three good layers. You can also get hollow pre made paper mache' eggs and those work well. Then I would add my details on top of that with Paper Clay.  Paper Clay air dries and is light weight. I would put a bit of white glue on the paper mache' egg before I started adding the Paper Clay details.  This helps to make sure the paper clay sticks.

The golf ball should work for you neck but since it is a monkey and I assume you are covering in fur, you could just make the connection string/soft rope.  

Your idea for the body is on point.  You really don't need much in the way of a wood frame.  It could simply be a block for the shoulders and then a block for the hips.  Tie the two together with cloth or leather. On the shoulder block you add some foam front and back to create the shape you want and the same on the hip block.  Try not to have to much padding in between the two so the torso stays flexible.  Hope that makes sense. 

A lot can be done with batting like you find in quilts to help shape body, arms and legs and it helps keep things flexible and light.

Always think about weight when you are building a puppet.  Remember you may have to be holding this weight up for a long time.  Take a can of beans and hold it at arms length in front of you for a few minuets... you'll understand why I mention this.