Here is a thought for you. 

Create a master portal and install a module for the content of the newsletter.  The module you install would need to have RSS availabe.  I would use Ventrian News Articles (www.ventrian.com) or the the core Blog module.  In this master portal you would manage your common content.

Create your first Agent's site.  Get it looking exactly how you want it to look using the RSS NewsFeeds module to put the newsletter content (rss feed from master) onto the Agent's site.  Once you have this portal looking good you can make it into a Portal Template in Host Settings.

Create the rest of your agents sites using the template you just created.  Now you will have 100 independent portals all with the content from the master.

For sending your newletter, you may need to have someone make a module for you.  DotNetNuke may not be the best system for sending large amounts of email, you may want to revert to a mail server with mailing list capabilities or a service like constant contact.  For a talented programmer, it would not be hard to have a module that you install on every portal where the agents can update their own lists.  Now you would have a single table with all the addresses from all the agants and you would also have all the URLs of the portals that own the lists. 

I would turn on friendly URLs so that all portals would have the same page names as opposed to tabid.

You could try DNNGurus.com

Jeff