System fail-over and redundancy. What is the best way to make sure that our Domain Controller Servers are always up and accessible, even when one of them experiences a catastrophic failure?

We are currently running two older Dell servers running MS Server 2012 R2. We are now getting ready to replace these with a Dell 1850 PowerEdge and a 2850 PowerEdge server(s). Can I please get some recommendations as to how to set these two servers up for backups and fail-over redundancy? They will also be running MS Server 2012 R2. How compatible are these two units together and is there a preferred way to set up backups/redundancy ? Any help from some of you more experienced Dell techs would be GREATLY appreciated. Thank you for your consideration and taking the time to give some badly needed advice.  : )  Ron.