Scripts intergrating so can be managed as 1

Hi, At the moment we have several scripts which all have to be edited manually each. I would like a system so that we can manage them all under 1 admin panel. We have 6 separate sites but they all use the same coding (well its 99% the same minor differences, like text on the page different). They are all hosted on the same site. I want to be able to manage the pages for the site and the email content it sends out under one admin. So: 1) all the HTML files the site has in the main directory you should be able to edit in the admin panel for each site. 2) create and edit HTML pages in the admin panel. 3) the email it sends users when they sign up should all be editable in the admin panel. each site has a [login to view URL] page e.g. [login to view URL], [login to view URL] etc. Thanks. Let me know if you have any questions.