Membership and ProSites integration for blog creation

Let me describe what I want to do and then, I hope the solution(s) will be clearer. Thanks! I do appreciate your help!

(BACKSTORY: Multisite, Buddypress, Sub-directory, ~~ is that all to say?)

I want people to sign up for a Membership on my new site. I would like them to be able to choose their desired level of Membership -- with the main difference among the levels being the number of sites they can create -- at this point it is CREATE ONE BLOG or CREATE TWO BLOGS, or even CREATE ZERO BLOGS. If they want more then can contact us....

I would like them to be able to sign up, pay the paypal, and then be returned back to my site in order to create their sub-site(s). Then the NEW BLOG TEMPLATE plugin will duplicate the settings that I have established in their new site(s) - fyi this part is working just fine.

Tangentially, I can see that we would want to add ProSites to allow the option of increasing the storage space for member's site(s).

The questions I have are:

1) what is the best setting of the ALLOW NEW REGISTRATIONS option in the Network Settings to allow this?

2) Yet, when I enable LOGGED IN USERS MAY MAKE NEW SITES, and then I go to (whilst logged in as subscriber on the main site, with a membership level allowing the creation of two sites) CREATE NEW SITE on the MY SITES page, it redirects to the BECOME A MEMBER page that I set up for the registration process. That doesn't seem to be the right page.... it isn't letting me make additional sites....

3) when this all works correctly, how would a member upgrade the capability to add more sites? In some sort of upgrade process, likely, but how do I make it where they are charged correctly for the additional membership level?

I think the answer may lie in ProSites, but I thought that the Membership plugin dealt with the person, and ProSites dealt with the site... And now I see that may be an incorrect or incomplete understanding.

So, if REGISTRATION IS DISABLED at the Network Settings, does ProSites allow a user to make a new site?

Please pardon if this is unclear. And I thank you for any and all assistance!

Stephanie