Backend and social integration for a game

Hello, we need the backend for a game using [login to view URL] and Unity. The game is multiplatform, it will run on iOS, Android and WP. We need you to implement and optimize the comunnication between Parse and our client program made with unity. You will program the server side and the changes needed on the client side for making it work. The basic functionalities are: -User management. The player will have assigned a persisten and unique userID -User progression: The progression will be saved on server when possible, in other case will be stored on the client till a connection will be available. It's a game based on levels (Worlds) so after each level we will have to save the progression. -Social features: Implementation and management of friend invites and list. We will also require to implement the social basic functionality on the client like share. Facebook and Twitter are mandatory. Yandex and Google+ are under study. -Achievements: The game will have achievements based on the progression of each player. -Leaderboards: Each level has a leaderboard that is based on the friends of the player. -Purchases: we need to implement a complete system for purchasing items with real money and with virtual money. The library that is required to use is OpenIAB. We need a system for verification the purchase and to recover them if the user login in other device. -Version and config control: We need to verify that the version of the config files of the game are up to date when launching it and if not get the new one or get any new content need it from a given server. We will spicify the fields of the config but keep in mind that will support A/B test cases. The code should be optimized for sending as few request as possible per user or to have a system to distribute the requests according to the server load. We are open to use other server but we prefer to use parse, if you have a proposal for other provider just sent to us an estimation of the monthly cost of the server for 50K DAU. If you have any questions just feel free to write me. We will require to have a demo of similar jobs before deciding the winner. Budget should be around 1K USD.