As for having forums for only paying members, you can setup groups that have access or not.

Any payment stuff, timed access, subscription notices has to me made. I don't know if any of this is slated for ubb 7.3, else you would need to ask again at ubbdev.

The banners for non-paying members could be done with the same group stuff. You can create code in the header (or any php insert) to check out the users groups, and display stuff.

[Linked Image from siemons.org]