Salesforce REST API

Hi guys, We're a mobile app developers. Need help to build/code public REST API with documentations for our existing website so that we can integrate the data with our native mobile apps. There are already same APEX class with similar functions on the website. So, we need to turn this functions and exposed it as public REST APIs. There are about 17-20 functions that need to be developed. I think it should be fairly straightforward and easy for experienced Salesforce developer guys. Let me know: 1) If you have done similar tasks before 2) Example of native mobile apps that you have successfully have done the integration 3) How long will it take to complete this tasks