Help with configuring a private site

I’ve installed the core WordPress, no problems getting going on both local and hosted sites. Excellent installation package. But am having getting started on trying to tailor for my needs, finding out which plugins I’ll need. I’m off to a bad start by having a requirement for a private intranet-like site. Where, without being logged in, the only page accessible is the logon page (maybe an external welcome page).

My need is a collaboration site, where I collaborate privately with multiple clients. The clients should be unaware of each other, members need not see each other, only the admin. There is no self-registration. The admin can set up all users. The set of pages created for each client is private to them alone (and the admin). There is no need for any shared/public pages or blogs. The client has certain privledges, such as creating their own private pages, uploading images, creating their own gallery and albums, uploading/downloading documents, blogging which is more of a personal diary.

So while clients/members have much of the usual wordpress functionality, they do so in complete isolation to each other. They can not see each other as users, nor see any content others create, blogs or pages. The admin (or supervisor) of course can see everything. Can send private messages to members, can add or edit content for specific members, but no need for shared content (at this stage, don’t need user groups at the moment).

So this should give a good outline of what I’m after. The question is, what Plug-Ins will give me this type of environment? Any advise for setting this up? I’ve tried a number of plugins so far, but none seem to be giving me the isolation I need between users, whilst giving them the ability to create some content (pages, blogs, albums, etc).