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How about setting up the site so that users have to activate their account via e-mail.
Change the template of the activation e-mail so that it contains your verbiage and a link to your paypal subscription checkout.
Once their payment goes through, you can activate them manually.
Unfortunately, I am unaware of any free or gpl modules/hacks that will auto-manage your membership database as well as your paypal payment history at the same time.
Anyway I noodle it out in my head, it requires work by the site admin.
Were you able to get the donations module to accept a donation, list the donation in the block and assign the user to a new group?
If you were able to get that far...your more than halfway there.
One idea would be to redirect people registering to a payment page(paypal subscription/submission form), and use a redirect from paypal that sends them to the actual registration page.
Have admins activate the accounts...that way, whenever you get an email from paypal alerting you of a payment or a cancelation, you can activate and deactivate the user.
This will also give you a minute to enter the user id and billed customer info into your own DB for tracking etc.
fun problem