Hi DarkCorner,
I know the joys of zero budget IT well. You clearly have enough common sense to ask for advice and the drive to do the best job you can so I suspect you will move on to bigger and better things in fairly short order. Make sure you don't leave the next guy with a solution only you understand
I second HannesK's comment about low end NAS. If you have sufficient Linux skills to manage it and if you also have somewhere else to run the VBR server, investigate the Linux hardened repository. If not, stick to a Windows PC. If there's any minimal budget available, look at a real RAID card and 4 disks in RAID10 for best speed and some resilience. You may even find the desktop can do this with the onboard controller. Second hand PCIe cards from enterprise servers are a good bet, something HPE or Dell but watch out for physical ports as custom cables are often used.
Do not connect your repository to the domain (assuming you have one) and instead use local accounts with strong passwords to manage it, NEVER browse the internet from it and if you have the ability to limit it's access to the internet, do so. Turn on the Windows or Linux firewall and lock it down as tight as you can, both inbound and outbound. Document your rules!
Again, if there's any budget, look at the VMware "vSphere Essentials Kit Term" license as a reasonably priced upgrade for your ESXi host:
https://store-us.vmware.com/vmware-vsph ... 20900.html
If there's a chance you may expand to more than 1 host in the future look at the "vSphere Essentials Kit Plus Term" license which brings DRS (auto balancing of VMs across hosts) and distributed switches plus some other nice to have features. These kits are limited to 3 hosts and fixed term licenses but represent huge value when compared to standard VMware licencing. You'd also have support to back you up if you run into problems.
Also consider Windows Server Standard licensing for your ESXi host and repository. You can run 2 VMs of Windows Server Standard for 1 physical license. If you keep the software assurance current they can also move around between hosts in a cluster. There aren't many ways to run virtual Win10 and stay compliant with Microsoft's licensing terms....
Best of luck,
Chris