Windows DFS File Server

I want to implement one DFS file server.
Daily user access is 400-500.

My question is:

  1. Which raid configuration should I do for this server?
    —- I want to make only 2drive. One drive is for server OS and other drive is for shared files.
    —- I want keep the hard disk expansion option for future. But, When I will add more physical hard disk, this will add this disk space to that shared drive. Don’t want use this physical hard disk as another drive in the server.