For learning curve I'd try Wings3D. If you've never modeled before you just can't start with anything any easier :). The objects from Wings transfer over nicely to Poser too. Perhaps after you've learned to model you can think about stepping up?

IF you already know how to model, then Silo is a good choice. Not very expensive, powerful and makes Poser-friendly objects.

You could also try Blender (which is free) if you don't mind pulling your hair out now and then. However, it does do just about EVERYTHING. ;)

Laurie