Depends - if it's a server from the big names like HP/IBM/Dell, you probably won't be able to replace the motherboard as they normally use non standard size boards that often only fit one particular model and may have strange internals like riser cards for pcie or ram, daughter/mezzanine boards for remote access/raid or strange power connectors. If it's a Supermicro or whitebox server you may have better luck but I would still look into it carefully if that's what you plan to do.
I doubt you would need 2 x i7's for your intended purpose, but once you have a vm host running, it's very easy to start thinking about how many more vm's you want to run.
Another thing you may want to consider now before putting the plan into action is memory. VMware is better if you wish to overallocate resources but isn't part of the syllabus while HyperV is. If you are using HyperV you will probably need a bit more RAM.
I believe you can run HyperV nested inside VMware, but you need a server with SLAT features which definitely rules out the dual quad and I have no idea what the performance would be like. Also check whether RemoteFX is on your syllabus as that requires SLAT.