I haven't used the oldest versions, so I can't speak for those. Poser 6, which is what I use currently, is a strong improvement over 5, so I would certainly use at least version 6. Supposedly version 7 adds some new features that sound mighty interesting--like multiple overlaid timelines with motion blending--but I haven't tried it yet.
Dennis has some awesome AC3D plug-ins useful for creating characters and some Poser-specific stuff that you'll want to get. (Whereas AC3D only creates models, Poser only animates, so you'll need both to make original characters.) You can download Dennis' plug-ins at:
http://supercoldmilk.com/