Technically you can do this because - 1. SharePoint is compatible with the ASP.NET MembershipProvider, which is where *most* of the user functionality you seek lies. 2. SharePoint has lists and document libraries which you could leverage for the content functionality you mentioned. Do take note of some of the strengths and weaknesses of sharepoint though - even if you plan to use the "free" version.

Sounds like the master file may be changed. If you want to revert back to the out of the box version, in sharepoint designer - right click the v4.master file and select "Reset to site definition".