How much configuring do they need? It doesn't sound like you are setting up an awful lot on them.
You could for instance use
Fog tp push a sysprepped demo image and post deplyment it can invoke a powershell script to do simple configuration. Reinstallation is then just a case of pixie booting the vm every couple of months. That might be enough to get you up and running without the complexity of Docker/Vagrant, DSC or Ansible.
But you can equally just buy a handful of one time Windows keys off Amazon for £20 each, and those are fine. That's probably what I would do.