User permissions question

Hi All,

I am thinking about using modx for a new project, but I can't think how to make it work.

Here is the brief: We need to have a site holding files only available to users with access to that file.
Login required to view any part of the site, and files will be organised in categories. Users may or may-not have access to those files, (some files everyone will be able to see) all files will be uploaded by the admin via the Modx manager.

As an extra, users will be able to upload files for the Admin to see.

Any help on this will be more than welcome.