This is far easier than you think. Allow anyone to sign up, change the registered permissions to nothing and then have a notice which directs to user-upgrades and allow them to "unlock" the forums. Apologies if you've already thought about this, but it'd be a better way to do it since you're giving them a sneak peak
The hardest part is PayPal in general, I think you mean - I'd stay away from them if it wasn't that everybody used them. Business accounts are simple, though.