Windows Server 2012 R2 Essentials and SQL Server 2014
I would ask your vendor why they can't install SQL Server on a domain controller because the documentation seems to indicate otherwise.

See the Installing SQL Server on a Domain Controller section of Hardware and Software Requirements for Installing SQL Server 2014


  For security reasons, we recommend that you do not install SQL Server
  2014 on a domain controller. SQL Server Setup will not block
  installation on a computer that is a domain controller, but the
  following limitations apply:
  
  
  You cannot run SQL Server services on a domain controller under a local service account.
  After SQL Server is installed on a computer, you cannot change the computer from a domain member to a domain controller. You must
  uninstall SQL Server before you change the host computer to a domain
  controller.
  After SQL Server is installed on a computer, you cannot change the computer from a domain controller to a domain member. You must
  uninstall SQL Server before you change the host computer to a domain
  member.
  SQL Server failover cluster instances are not supported where cluster nodes are domain controllers.
  SQL Server Setup cannot create security groups or provision SQL Server service accounts on a read-only domain controller. In this
  scenario, Setup will fail.
  


So there are a few limitations, but I don't think these will apply in your situation. Using a domain account instead of a local service account shouldn't be a problem and I don't think you will use a cluster if you only have one server. Your domain controller probably isn't read only either.

The process is also documented in the Autodesk Vault knowledge base so I suppose it's supported for at least some of their software. 

Microvellum's documentation seems to suggest the installer will check and you must not install SQL Server on a domain controller, but in fact that will only generate a warning but still allow you to continue, and it will work as long as you don't try to use a built-in service account.

Windows 2012 R2 essentials is also a supported operating system if you look at the Operating System Requirements on that page as long as it's 64bit.

That being said. If your vendor insists, you don't need to reinstall the operating system to remove the domain controller role from the installed operating system. The steps to demote a domain controller are documented.

Be warned though, if your workstations use a domain account to log in to the network and you remove the domain controller, your users may lose access to their PC's and the network, so you should be very careful when considering that route.

Also note one of the limitations of installing SQL Server on a domain controller is that you can not demote or promote (i.e. remove the domain controller role, or make your server a domain controller) once SQL Server is installed on the machine. In order to do so you must first uninstall SQL Server and then reinstall it afterwards.

As a last note, if you really can't install SQL Server on your machine because it's a domain controller, and you still need the domain controller role for client authentication, you could consider running a virtual machine on your server and run SQL Server inside that VM.

You should always verify licensing with your vendor, but this seems to suggest it could be a viable option without having to purchase a new server or an additional license:Understanding Licensing for Windows Server 2012 R2 Essentials and the Windows Server Essentials Experience role


  The Hyper-V role and Hyper-V Manager console are now included with
  Windows Server 2012 R2 Essentials! The server licensing rights have
  been expanded, allowing you to install an instance of Essentials on
  your physical server to run the Hyper-V role (with none of the other
  roles and features of the Essentials Experience installed), and a
  second instance of Essentials as a virtual machine (VM) on that same
  server with all the Essentials Experience roles and features.