SAAS with payment after 15 days

Hi !

First of all, thank you for this great website !
I speak french but I'll try to explain my needs in english. Be indulgent :p !

I'm trying to set up a multisite service, like on Edublogs.
1. On the main website (call it http://www.main.com), I want the users sign up and create their own website with all the features available for 15 free days.
2. After these 15 days, the front end has to be invisible for the visitors until we receive the payment. Like on Edublogs, I want that the users carry out the payment inside their admin panel.

Wich plugin(s) do I have to use ? Pro sites ?
Could you please give me a step by step guide to do this ?

Thanks