This sounds like a two NAS solution.
I have setup similar situations for home-office and small-business use with up to 30 users.
In fact I even do this at home for the 4 Windows machines and 3 Mac's me and my girlfriend have lying around.
First one as the server: With 2 disks in mirror or with 3 or 4 disks in a raid5.
Raid is no substitute for backup so you will need a second NAS as backup of the first.
This one just needs 1 disk (if it fails you always have the original copy on the other NAS) but is preferably located in another fire-zone in the building.
(In my personal case it is in the garage which is separate from the house.)
Go for Synology or QNap. Those are the 2 top-brands and support this right from the box.
They will run SMB for Window clients and AFP for Mac users. Macs's can do SMB but are much happier with AFP.
(And with AFP the NAS can be used for TimeMachine backups for the Mac clients too.)
Even buying 2 NAS boxes is cheaper than buying 1 Windows server with the Windows license. (And you won't get AFP with Windows. )
And you would still need a separate backup solution for the Windows box.
Setup and maintenance of a NAS is MUCH easier than a Windows server.
You don't want to babysit the Windows server every patch-Tuesday do you ?
PS: Buy 2 or 3 TB disks. It may seem overkill now but diskspace usage tends to grow exponentially and disks are cheap. 4 TB disks are still a bit pricey.
PS.PS: If you want versioning of your backups the quick and diryt approach is to don't use the default rsync backup provided by the NAS (which makes 1 on 1 copies), but run rsync from crontab on the 1st NAS with a different target folder for each day of the week (or whatever other versioning scheme you want).
There are also all sorts of more complicated backup-solutions that you can find on the internet. Many of these are also available for your NAS of choice. After all they are just Linux boxes with an easy to use interface as front-end, so just about anything Linux based can run on the NAS.