Here is my 2cents:

You do not really need to buy anything at the beginning.

Just use V4.2, the basic lights (start with one, you will see how it can be great!).

The most important thing is to not use the preset renders Poser is proposing. They simply sucks!

You can try to re-render the same image you have posted simply changing the Renders parameters:

• Use Raytracing

• Use displacement maps

• increase pixel samples

This options will help to have better definition of details, such us skin and shadows.

It is important to know that the standard skin of V4.2 is not high definition, so it will never seems like real skin.. but it can be good to start.

Learn how to position lights, and how to use them at best. There are lot of tuts here around bout lights.

Sometimes I find useful to set the Texture filter off. This is good specially with hairs. To do this you have to use a phyton which is included in Poser (file - run phyton) under the runtime folder (search for it, is called TextureFilter.py).

I am far to be a pro.. but I think that you can really improve from the beginning with some small tips.

Ciao!