I was going to suggest that something this small, without any professional services, might be a good use-case for Azure. I can't imagine it would be very expensive for something basic like AAD and no hardware to deal with.
I suspect it would surpass the cost of an inexpensive server before the server would have depreciated out. Eh, I'm not much of a cloud fan. I've made some real friends in the industry by telling them that the cloud exodus is coming so don't send to much of your shit to the Hotel California.
Now to figure out how I can delete $67,000/month worth of backup files. No one can identify why they were there, who did it (done under the account that opened the tenancy, person responsible long since gone), or any other useful information.
That's the power of the "cloud"
Back on topic. You could just pick up a small HPE Proliant tower with a basic RAID 1 array, 8/16GB of RAM, an E5 CPU, etc. Very vanilla but will probably run forever if it's just a DC and no one dicks with it or uses it a personal porn station.