G'day mate,
I've done plenty of 'balance' based scripts, and this would very easily be an implementation of that same type of system (although the balance would be replaced with credits obviously).
That said, I have no doubt I can deliver a product that you will be more than happy with, and that will give you the most chance of success. The end product will be 100% custom code (apart from several 3rd party plugins such as ajax and bootstrap), which you will have all of the rights to do with as you please.
I would start by building the entire site and ranking system (profiles, friends, following/followers) and all user management based things (avatars, change password, email, invite other users). Following this I would implement a paypal based payment processing gateway so you can automatically handle payments for credits within the site. These transactions and users and such will be 100% plugged into an admin panel in which you will be able to control every aspect of your site within reason.
I have plenty of experience building large scaleable from scratch custom software's, and I am more than confident that I can build yours to spec. I would love to hear from you so we can discuss the project in further detail.
Thanks in advance mate,
Alex