PHP - website billing

I am looking for a middle PHP developer for the project, studios and signors - do not disturb.

We have a Saas service site that sells several different subscriptions to our customers at once. Individuals and legal entities. To facilitate the architecture of the users personal account, we decided to allocate the subscription manager, payment and everything related to this to a separate subdomain, where the user goes from his personal account.

Given:
The billing code works well with two payment gateways (for jurists and physicists separately).

Have to:
Greatly change and simplify the structure for the future version of the personal account. Implement a bunch of features, according to TK:https://docs.google.com/document/d/188sHNtEra5O5HH...

Conditions:
In order not to dig much at once in someone elses code, I strongly divide the task into many stages. Payment is also phased,

I expect from you an hourly rate and detailed experience of both the backend and the front, if there is