Help in making a multi-platform application using Unity

Hello Everyone, I am making an application for android and iOS devices, The application needs following features : 1. Its own user registration and login system. 2. Able to store Text strings and Photos on cloud. 3. able to retrieve those text and photos. 4. Offline data persistence and synchronization. I want the app to be created in one place and be able to export to all mobile platforms[ android , iOS, windows phone] so i have decided to use unity, But i am not sure about which cloud services i should use, so far FIREBASE seems the best , but they dont provide any official sdk for Unity. I need someone who can help me making the app, I will code and create the complete app , i just need a mentor who can help me with the cloud stuff, Your job will be to chat with me and point me towards the right documentation where i can find the answer to any problem i might have. Please bid how much you will charge for 1 month of such support, And if you have read the description then make the timeline in your bid as 30 days, Also please mention which cloud service will work best with unity, and if you want to suggest any other engine than Unity then please let me know, Thank you very much , Looking forward to start the work soon.