Drupal Community Portal

I have a project that requires using drupal to build a multi-site with individual domains. This site needs to have a splash page with links to each sub site, have one login screen and run on the same db. Each site needs to have it's own unique theme and content as they are different businesses under one group. I would also need a way to filter content in the administration pages as to which site the content belongs to. Is there a module that would help make this easier...? or a combination of modules? Anyone with prior knowledge of this kind of project and solution please share your experience. What is the best way to plan this project?
Thanks