If you want to run it on top of Windows, check out VMWare Player (free) or Workstation (100-150). Do take into account that Windows 7 will take resources in addition to the VMs you install on it. That's why bare HyperVisors are often recommended. However, if you decide to go this route. I don't think it's worth it for you to spend 700 on Server 2008R2 to have Hyper V with a GUI and then buy SBS Essentials. More cost effective to use Windows 7 and SBS Essentials if you want to go that route.

While running a bare hypervisor is more complex, it's more efficient and more secure since there are fewer attack vectors and the footprint of the hypervisor is small. ESXi is free and Hyper V is free. The backup solutions can be pricey though.