Direction and setup of my idea

Hey all

I have been deliberating for quite sometime on the best setup for my new project but still cannot set on a clear direction and setup. Therefore I thought the experts over at wpmu dev could help me out and any other willing wpmuees!

I would like to offer my customer the opportunity to sign up for a new website on my multisite install which will have a selection of features the essential of which are a league manager (i have the plugin) and a buddypress social network for the memebers, while still having additional wordpress pages (some limited by registration.

I am looking to setup my wordpress mu on a single domain, lets say http://www.example.com. Then each new customer can purchase1 of 3 different levels of packages, the top level being the chance to have it on their own domain.

Does each new site need to be on its own network in order to have its own social network(buddypress) (that does not share users with anyone else on the multisite install)?

Whats your thoughts guys on the best policy to set this kind of thing up, kinda what process/plugins would you follow?

Many thanks