Is WPMU what I need for my web franhise?

I currently have a large portal web site and will be creating a "web site franchise" for portals in other areas.

Each portal will look the similar but with localised content and I am wondering if WPMU is the easiest solution for what I want to do.

So when we create a new franchise for a new area, I guess we will click a few buttons in WPMU admin and create the new site. We will then give the "franchisee" of the new site login details and they start plugging away at generating their content.

I am a heavy WP user and am thinking that WPMU could help me achieve my goal of making the administration of the 100 or so franchises a little bit easier than maintaining 100 individual WP sites.

My other question would be:

If I create a plugin for WPMU does the same plugin filter down to all the other blogs? (the idea is that we create a special plugin for displaying train time information and allow each blog user to edit and display time on each of their blog)

Thank you kindly in advance for your responses.