Of course you can promote to DC. It can run a standalone domain with no machines. It just simplifies EVERYTHING, including "just" exchange/retain files, using it as a "workstation" doesn't simplify anything. You can uninstall RDS and JUST use RDP to connect to the machine using local users, but if you want to use RDS functionality, you'll need a domain environment.
Just recently got a new PowerEdge T430 and along with it came MS Server 2012 R2. This server is going to belong to a workgroup with users remotely accessing it. My issue at the moment is this message: you are currently logged on as local administrator on the computer you must be logged in as a domain user to manage servers and collections. Am I missing some setting? Or is this a limitation of 2012? Will downgrading to 2008 resolve my issue?
Windows Server 2012 R2 - Remote Desktop with single server