1) When you mention "free" Hyper-V, do you mean on a Windows 8 system, or the Hyper-V server core? The Hyper-V server core is great, but I don't recommend it as your first take at Hyper-V as it requires remote admin and is more difficult to set up since there is no GUI. If you meant Hyper-V on Windows 8, just remember that the virtual machines (VMs) are only as stable as the base OS. I.E. If you will run a domain controller (DC) as a VM, I highly recommend you leave the base OS (Windows 8) as clean as possible.
2) You can always install TWO domain controllers (on separate machines, preferably...) They can both be VMs. That way if you have to turn one machine off for repairs, it crashes, whatever, the other DC will run the domain. You can do this on two VMs on the same physical machine, but this only helps if you have to "reboot" one of the DC VMs. Obviously if the host machine is rebooted or crashes you're still out of luck this way. You can have the more powerful machine do the transcoding and file hosting as double duty while a low powered machine acts as DHCP and DC only.