Building a WordPress MU membership site – need suggestions

I’m planning to develop a WordPress MU (soon 3.0) membership/hosting site, and I need suggestions. I will be using WordPress for the CMS capabilities and back-end architecture. Users that sign up for free blogs will have 1 theme and 1 plugin set as default (unchangeable), and users that pay for an upgrade will have a different theme and plugin set as default (unchangeable). I want the administration for the users to be very simple, and to be all controlled on the front-end (completely restrict wp-admin access). Users will only be able to: post, edit posts, delete posts, comment, moderate comments, delete comments, edit comments, add and edit tags, and adjust some basic settings. The P2 theme allows for some of this already, which I will build upon and add a “settings” page to adjust some of the other features.

What would be your suggestions for the best plugin or system for establishing a membership site with these capabilities?