We are a small business of two, specialised in Unity and interactive apps.
I would not trust a developer who does not tailor the solution to you exact needs but to do that requires more details. So instead i have answered each step with some suggested considerations:
1. FBX files will already work in VR if you know what you are doing so perhaps i misunderstand this requirement?
2. Similar to 3, you need a networked solution. I assume you mean the content should work in a web browser? Typically you can look at WebGL but depending on your graphic requirements we may need another solution.
3. If this is not a game and you do not need to worry about cheating, a peer to peer Photon solution would be the cheapest and most effective.
4. uGUI Is the current complete solution for this but its not efficient. There is a up and coming UI Toolkit solution that may be worth looking at if your models and visual requirements are demanding
Our recent complete client projects have involved updating a relationship simulator to no longer rely on Flash and making an mobile app prototype for helping supermarket consumers improve their purchasing choices. My profile shows a VR project built in Unreal Engine for Hamburg University Of Music And Theater and a motion camera and projection project made for Legoland.