Add Memebership Tiers to Registration Form

I currently am building a WPMU Buddypress site and as part of the site I have a post quota system with five different tiers of users. I currently am using Gravity Forms to do the user registrations because my client wants a unique flow for the site - however I'm wondering is there any way to add a field to the registration system where I can assign a user role based on a one-time payment?

Also I'm wondering if I can update permissions down the road (i.e. upgrade/downgrade users manually - or automatically - whichever is easiest) since right now the only permission control I can find is to give users super admin roles which is not close to what I need.

Also I was looking at the Pro Sites plugin but unfortunately I can't use it in this case because I was having a lot of issues with other plugins and I also need post quota support - I'm willing to give it another shot if it can do what I need here however.

I think the reason I dropped it is because my client wants a one time payment rather than monthly installments for the service.

Thanks in advance for any assistance,