Pro site: Offer free blogs only

Hi!
I would like to offer my clients the possibility to create websites/blogs for free only. They should only pay for premium plugins or premium themes.
Is this possible with Pro Sites?
I also intend to become a member and use all necessary wpmudev plugins to make this project possible.
Some detailed explanation on how to realize this project as soon as possible would be very helpful. Thank you !
Regards,
Dita