New to Drupal -- Need Some Help Getting Started

Hello. I am new to Drupal. I have gone through the O'Reilly Using Drupal book and worked all the examples so I am a little familiar with what Drupal can do. I am designing a website from scratch that needs to have 3 or 4 pages for the public and then a log-in to a member area. The member area will have its own "home page" with links to groups pages. There will be 6 or 7 groups that will be using their site to maintain content that will be made visible to the public (i.e., show as a summary on the public pages) as well as maintain documents that only members of the group can collaborate on, see or edit. I'd also like to be able to maintain a member directory.

A few questions about design before I get started:

How do I make the public web pages different from the member pages?
Should I use Organic Groups to facilitate the group collaboration and editing?
Should I make use of Wiki-like pages instead?

Thank you for your help.