Hyper-V server is available for free use. It would be a solid choice for the physical server as it would allow you to host many VMs. The only licensing you would have to consider is that necessary for the guests (VMs).
In larger environments the Windows Server Datacenter license is often worth it, but you'll have to do the math yourself. How many Windows Server 2012R2 servers do you need? What's the cost of their licenses vs the cost of a Windows Server 2012 R2 Datacenter license? Note, each Standard license supports up to two physical processors per host. If you have a server with four CPUs you'll need two licenses.
Download Windows Server 2012 R2 Hyper-V here:
Getting started steps for Hyper-V:
Because this is for business, ensure you're backing up your physical server regularly. Consider adding a second physical machine and setting up clustering. Even if you only install Hyper-V on a desktop and without clustering it, using it to host a second Active Directory Domain Controller (ADC) is well worth it and will save you if something goes wrong with the main hypervisor. You'll be able to host a few VMs on a desktop if you have to, but don't overload it.
Your Microsoft approved reseller will be able to help you with all this stuff.