Wordpress MU Membership Setup

I wonder if you could please offer some advice on the best way to setup membership on a Wordpress mu, based upon the following scenario.

All of my sub-sites are subordinate to the main site. Each of the sub-sites is dedicated to a specific subject matter.
However, I want potential subscribers only to register and pay via the main site, but have access to all of the sub-sites as well as the main site.

I have a plugin to replicate members across the network and that is working great.

The entire network site content will only be available to paid members, bar a few information pages.

Thank you.