I am quite confident that I can rig the figures (models) in Poser - and using other third party products - make suitable textures and material groups. 

As far as selling , well, I have seen things done in Poser 4 / pro pack (the old curious lab version) that is just as good and in some rare cases even better than Poser 6 stuff that I've seen.

I was planning on making free stuff - but I doubt that if were talking about some really awsome models - then if they were at an inexpensive price, I think people would like them despite the "oldness" of the poser file types.
Go to the DAZ site - DAZ sells PLENTY of stuff for DAZ STUDIO, including plug ins.
ALSO - converting an older style POSER 4 file to a "NEW" DAZ file type is not a problem. You don't always need the newest thing on the market to do some outstanding stuff. You can use the old stuff in the newer Poser versions AND DAZ STUDIO.

With the exception of a few enhancements and program features - at it's core even the new Poser is still the same, and can read the same file types it always has.
I don't see a relevant issue here...