All that can be done through membergroup settings.
For example. If a user registers at your site, they are placed in the "regularmembers group" by default unless you specify otherwise.
So if you don't want them to see your forums, then in "admin > boards > modify", unchecked the regularmember group. BY doing this they cannot see those boards.
Then with your subscriptions group, make sure it is checked. That means anyone in the subscriptions group can see the boards.
Then with the subscriptions mod, set it up so that anyone that buys a subscription, is automactically placed in the subscriptions group. This will give them visibly and access to your boards.
EDIT: also, you can do this with everything else. Like if you don't want those who haven't subscrbed to see/use teh downloads manager, just uncheck the perssion for reguslar mamabers, but checked for subscribers.
You can also do more than one subscription group with higher paid packages, etc etc.