All legacy content except for python scripts should work with Poser 7.  Any figures you buy on Daz 3d are fully compatible.  For some fun Poser trivia:

Dork = P4 Male
Posette = P4 Female

Pre poser 5 figures are low res compared to the new figures, they are good for background figures, animations or cartoons where you don't need a lot of detail.  After Poser 4 Curious Labs gave the native poser people names (I think in part to personalize them and in part to compete with Daz's line of excellent figur).  You'll still hear the old figures referred to as Dork and Posette.

When you get your software take time to go through the content library and see what you have.  There are some 3rd party figures provided, and some excellent new animals.  Be sure to download the free items from Content Paradise once you activate your membership!