DC on Hyper V Host

While the scenario you describe would work, the "best practice" approach is to not run any roles on the Hyper-V host other than Hyper-V; all services should run on virtual machines running on the Hyper-V host.

The hardware you describe should be more than capable of running three Hyper-V hosts; especially since a dedicated DC for 13 users would require minimal disk and memory resources.

The best option would be to obtain a single Windows Server 2008 R2 Enterprise license; this would entitle you to install the Hyper-V role on the hardware as well as up to four additional virtual instances of Windows Server 2008 R2 running on your Hyper-V host with no additional licensing cost.

If, however, you plan to use existing licenses for your application and file servers, another option would be to install Hyper-V Server 2008 R2 on the hardware to host your virtual servers. Hyper-V Server is free to download and use; however, no licensing is included, so any virtual machines you create must be individually licensed. Hyper-V Server is based upon Server Core, so the footprint and exposed surface area are very small. The UI provided with Hyper-V Server will assist in configuration of the machine for remote management; in this scenario, you will create and manage virtual machines on Hyper-V Server using the Hyper-V Manager MMC from your workstation. (Microsoft provides good information on this topic in the Microsoft Hyper-V Server 2008 R2 Getting Started Guide)

If I understand correctly you are talkling about hyper-v Server 2008 (not Windows Server 2008 with hyper-v role enabled). If so I think you will have no domain until you have the DC running within one of the VMs.

To administer the hypervisor (using Hyper-V manager MMC from your workstation) you will need to set up a workgroup to manage the VM's until they are running. It can be done (that's how I manage my hyper-v server at home) but you will probably need to use the HVRemote power shell scripts to get it setup.

Hope that makes sense.

Works, but I would NOT Move the file server to a vm - what for?

I have a similar setup(just 2 computers) here, and I use the DC also as file server. As all file acces is via DFS anyway (so the server name is never part of file access) this has no negatives, and I save having a VM just to provide a function that is core to the OS anyway.