1. This partly depends on how much you want to spend.  If you want to do Poser animation you'll want Vue 6 Esprite at least.  Unless you're using Maya, Max or one of the other programmes xStream works with you can save yourself some money because you won't need it.  Have you looked at the comparison chart on e-on's site it might help you see what the differences are?
2. As you move up through the different variants there is more as they have more features but the interface and the way things work is pretty much the same across all versions.  It comes down to how much time you have to spend on learning to use Vue.
3. Try JC's directory: Art Head Start he has lots of links to tutorials
4. Cornucopia 3d has lots of models for sale.  I've not got a models/material/etc category set up yet but there are a lot of sites that do listed in my vue links directory.
5. Mostly but not always.  EcoSystems can only be used with xStream Infinite, Pro and Esprite with the appropriate module added.  Python scripts (like SkinVue) only work with Infinite and xStream.  Some models may use features that are only available in some versions.  Hopefully if it is important it will say so.
6. Get all the chores out of the way before you start using Vue it can be addictive.

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