We are making a Multiplayer game using Unity. The actual game would be run from a PC projected on a screen and the players would be using their respective Ipad as a controller to play the game. I am not sure how difficult this would be to achieve inside Unity but I am assuming that all the game data inside Unity would need to be fed and retrieved from some kind of a server. How would this be possible with Unity because each Ipad would have a separate Unity build and also a separate build for the PC from where the actual game would be running.
Since Unity can publish games to iOS, this would be just like making any other client server application. The easiest would be to have the iPads all connected to the same network as the host PC, but you could easily utilize the Master Server tools provided with Unity to get everyone connected.
You'd likely use RPCs for most of your communications. As long as the PC build and iOS build defined the same RPCs (which they would if you don't change your code between builds), you wouldn't have a problem with them communicating.
As for how much game data you transfer between the clients and server, it will depend almost entirely on your game and how you've designed it.