Unless you have a high-end program like Cinema 4D, Maya, 3D studio, etc, you don't need Xstream.  It's basically Infinite with a plug-in to work inside the other programs.  Easel is the most basic and limited version.  Each version above that adds to what the program can do.  E-on has a chart on their site that lists the differences.  I use Infinite, and it really is a great program. 

The nice part abut Vue is that the interface is the same, no matter what version you use.  So, if you decide to upgrade in the future, you don't loose what you have learned.

http://www.cornucopia3d.com has lots of models, freebies, tutorials, and a good forum.  Lots of resources to explore.

Vue items are for the most part available for all version.  Sometimes though, a material or plant might use something that a lower version does not have, so it's kind of a yes/no answer.

Hope that helped.

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