What you need is SaMBA with all the options to use as a DC (Domain Controller). This was being used in a secondary school for at least 10 years here in the UK until 2012 when I lost contact and probably still is. From what I have read SaMBA has vastly improved and is far better even now in the DC role. If you read the latest documentation on SaMBA you will find all of the information you need. ( https://www.samba.org/ ). Most people think of SaMBA as a CIFS app to share documents between Linux and Windows but it is far more than that.
I ran Windows networks with mainly Windows servers including DCs, however I would not have an issue with using SaMBA as the DC though that does depend on your requirements. If you run other Windows applications from your DC then that may not be possible, as may some other requirements. However you maybe able to use SaMBA as your DCs and limit the number of Windows servers for the other applications and hence saving yourself money. SaMBA also has the ability to act as secondary DC to actual Windows DCs. What you could do is set up a test of SaMBA and give it a whirl before rolling out. I am sure that Jay will have some input as well.