Refining the application for Ios

There is already a working application for ios on Quick with a catalog of goods.
It is necessary:
- add a user's personal account (registration by email, entrance via facebook)
- Add the option of ordering and paying through Paypal
The server is operated by API +oauth. API in development, 90% ready
There are designs and requirements.
We are discussing the budget for payment.
For a job done well and quickly there will be a bonus.
What questions do I want to answer:
- Development experience for ios
- experience with git
- Experience with REST
- experience with oauth
Experience with payment services, particularly Paypal
- Facebook API