Heya @MrE
Your scenario is my scenario pretty much. My users are a 15 person software development company with no regards for past 20 years of development in any but our own software.
I have a opted for the following:
1x supermicro server with 2x 10 dualcores = 40 cores for virtualization and 256GB of memory, and a disk to boot from and store the config.
1x TrueNAS (or equivalent) Z20 with raid 10 over 10 disks, giving me 40TB to play with on a ZFS storage layer, exported with NFS to that supermicro proxmox server.
On top of that is running 9x NethServer, 1 as in this guide, a SOGo mailserver that connects to this one, a SAMBA File and VSFTPD FTPES server, various webservices on their own dedicated virtual server, and like 30 Windows 10 clients, mostly for VPN connections. I can virtualize most of our company without running out of resources, and am a verry happy camper.
Netserver can fill all these roles you mention, and even help with transitioning, as it is easy to set up and at no cost but resources on the virtual environment, you get to try a lot without much consequences.
A FreeNAS would be equally good, or the other one people promote lately… just make sure you have at least 10 drives in it, and use RAID10. Nothing worse then slow disk performance on a VM environment.
Use the cache options as well … we have SSD’s for cache, and otherwise just spinning rust, and outperform most VMware solutions we encounter at customers, while those cost a multiple of our environment.
So yeah, Nethserver can help you out, and I am writing the mailserver guide at a slow pace atm, but it will be there, and we run it in production already 
Dragged you here cuz the threat would be derailed by this answer 
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