The main concern with Windows 7 as your base instead of ESXi or another dedicated Hypervisor, is that of graceful recovery of VMs in the event that the host OS has to reboot, or suffers some sort of crash. ESXi pretty much never has to be rebooted unless you're changing hardware or applying one of the very infrequent updates that is released. Windows 7, on the other hand, would need to be rebooted quite a bit more often for updates (yes, you could turn them off, but that is risky on a system that is housing that much data). That means ensuring that it safely powers down all the VMs before doing so - without native OS support for it, that can be a tricky feat to accomplish in any sort of automated fashion. There is also the issue of passing hardware through to FreeNAS - I don't think you could do this in Windows 7, so you'd be forced to create virtual disks on those drives, and then have FreeNAS use those. While that would work, it does introduce another layer of complexity into the system.
Now, another option if you'd prefer to stick with the familiarity of Windows would be using Hyper-V as your base, though you'd have to learn how to manage it through remote tools or the CLI unless you have access to a full copy of Server 2008 (or 2012 when that releases next month), since the free version of Hyper-V does not have a native GUI.