How to accomplish this Simple Task. (What plugins do I need?)

I have a service that I will be charging for using Plimus.

The gist of it is, I will be offering websites of a certain template for users for a monthly fee.

I would like to automate this process as much as possible.

I am using New Blog Templates already, and I have registration disabled so that I can go in and create a new site and user each time.

I looked at several of the memberhsip plugins on this site but none seemed to do what I need

I need someone to be able to

1) Check if their subdomain is available or already taken

2) Only be able to make ONE blog per recurring payment of service

3) Specify their password and email when creating their blog

Their blog would be

It would automatically make them an admin of this site based off of the Default New Blog Templates plugin.

I do not need to lock down any content or anything like that, they can’t have access to the main site at all, only their subsite. But I do need to make sure that people have PAID before they are allowed to make a new site.

There is a Plimus IPN plugin that will generate users on a wordpress install when it receives an IPN, a solution liek that would be great, but I believe the plugins on this site already exist to do what I need, Everytime I look at them they seem to be more about protecting CONTENT rather than protecting the ability to build an individual site.