Start with free -- Wings, or since you already have Hexagon...   There may be more of a learning curve with some free software, but that's better than plopping down serious bucks then discovering you hate modeling.

Regarding the rest of the tool set;

"Clothing, Props, and Morphs" covers a lot of ground.

Let's start with props.  At their basic, these are a mesh and a texture.  You probably want a modeling application for the mesh (as Doctor Geep will remind you, you can actually model in Poser -- a little!) and to get the best texture results a photo editing software such as PhotoShop or The Gimp. 

That's all you really need for the basic prop.  Oh, and Poser or DAZStudio -- whichever one you wish to design for.  You can do this stuff blind, but it really is better to be able to open it and edit it in the proper application.

Oh, and a text editor can help a lot with some of the prop tasks.  Nothing fancy; Notepad or whatever you PC people use.

Clothing, conforming clothing, is a subset of "Figures" with it's own complexities.  You can do it with just the tools above but there are various third-party tools that will make it much easier.  Start with props, though!

Morphs are a subject completely on their own.  Whereas for a prop, all that matters is good mesh construction (no n-gons, no double-sided polys, stuff like that), to get a morph to work means you have to have a modeling tool that respects the number of polygons, the winding order, and the scale.  Disturb any of these, and the morph won't function (or, rather, it may ....if exploding heads was what you were after!)

Anyhow.  A few introductory words there.  Start propping and have fun!