Why go for Drupal 7?
I would start with D7 if I were you. Taxonomy and CCK are in core in D7 and Views is really stable in my experience, even though there's only a release candidate at the moment.

Do not underestimate upgrading a site from D6 to D7. Yes, it is possible, but you want to save yourself a couple of headaches and start with D7 right away. 

Drupal 7 is much more scalable and since you're planning a huge site, you're going to need that.

You write that you will be hosting documents; does that mean that you're storing the document contents in the database, or are you hosting downloadable files like .pdf's and .doc's? D7 is much better for managing files, as is explained in this Acquia blog post.