Well, the first step is not going to be "modeling and rigging for Poser," but "Beginning 3D Modeling"; it's not clear from your post whether this step has been done. With programs like Blender and Hexagon out there, that are very capable and very inexpensive (Blender, after all, is FREE), you're already halfway to your goal once you have that part taken care of.

This site has several tutorials for rigging models for Poser. You could also check out PhilC's videos on YouTube (http://www.youtube.com/watch?v=q8jxSOL_MAM). DAZ3D.com also has several tutorials on modeling, rigging, conforming, morphs, etc. that apply not only to Poser but to their own Daz|Studio and other modeling software.

Pretty much any modeling software that can export an OBJ file is usable; it depends upon your budget, your needs, and your enthusiasm. Poser itself is ready-made for rigging your model once it's built - I haven't used Daz|Studio, so I can't speak to its current capabilities, but it now has an advanced version.

Good luck! You're definitely in the right place. (BTW, PhilC also makes some great utilities, including one for making your own hair ;-)).