Custom Internal News System with Permission

Hello Just looking for quotes at the moment. I want to build an internal news system (similar to an extranet) with permissions. First screen the user is shown will be a log in screen. The news system will be split into tabs / sections, for example: -> General News (main screen shown after log in) -> News Category 1 -> Subnews Category 1 (1) -> News Category 2 -> Subnews Category 2 (1) -> Subnews Category 2 (2) -> Subnews Category 2 (3) The administrator should be able to add and remove tabs as he wants. The administrator needs to be able to add and remove users and set their permissions. Each user will have different permissions which are set by the admin for example: User 1 can view and post in General News. User 2 can view and post in General News and view BUT NOT POST in News Category 2 and all its subcategories. User 3 can view and post General News and Subnews Category 2 (2) Within each section the news will be posted in a forum style way, each news item will show: Date Time Title News Content (WYSIWYG editor) Also have the option to upload images and files. Administrator can delete and edit posts. I think there will definitely be some existing open source out there that can be used to do this and modified - but it is up to you what you use. I will provide PSDs once I have the project go ahead from the client you must theme to these (will be fairly simple). I need to provide the client with a quote before this project gos ahead. Thanks