Subdomain Registration

Hi everyone!

I want to create a WordPress multisite just like WordPress.com where user chooses a subdomain and enter the details to create the subdomain but with minor modification. I want to do the following:-

1. User chooses the price package

2. After choosing the package the user is redirected to signup page where the user will enter the details along with the subdomain name.

3. After that user will be redirected to Paypal, where the user can pay for the package

4. After the payment the user will be presented with the login page to enter into the dashboard of his subdomain.

I want to do it using WordPress multisite and for subscription i want to use s2memeber. So please help or give some idea how i can achieve this.

If anyone have any better idea please share.

Thanks to all in advance.