[Kirk Y]

Users purchase subscriptions through their UserCP. A "Forum Link" can be added to the Special Forum that will take the User to his/her UserCP to purchase a subscription.

When you're creating the subscription you can choose which Usergroup the user is to be added to upon subscribing. But you're also given an option to grant the user access to a specific forum; you can choose whichever route you want to go.

You can mark a payment as "recurring", which will automatically renew the user's subscription when it runs out. Otherwise, they'll just have to go back into their UserCP and renew their subscription.

You can place normal users in the Special Usergroup without being affected, yes.

Whenever your current users' subscription runs out, just ask them to renew by going to their UserCP.