Advice needed for best way to make money with WPMU

I have my WPMU install all setup and ready to go, but I'm not sure that I am going about making money the right way.

Basically, I want people to have 3 options:

1. Free site, basic themes, basic plugins
2. Pro site, extra themes, extra plugins.
3. Premium site, same as pro site but our people would build and maintain the site for the client.

And we'd also like to give clients the option to buy more disk space.

Right now I am using Support for options 1 and 2 and using Upgrades to allow disk space purchase. This is probably the normal way to do it, but I really find the "credit" system to be a little awkward.

I guess I'm wondering if there is a way to use a system like Supporter but ADJUST the subscription rates. Ie:

- if the user wants an extra 2gb of disk space, their subscription goes up by $10/m.
- if the user wants a managed site, their subscription goes up by $50/m

Can something like this be done with the Membership plugin? Is there another combination of plugins that will let me accomplish this?