Also, It is my understanding that ESXi (the free version) does not support a licenese to host comercially.
All of our servers are VMware. Running vSphere. Which is VMware's "Cloud OS".
Our control panel makes calls to the infrastructure using the VIX API.
DeadAIX.. AFAIK... The only hardware limitation even with the free version is that is requires a SCSI storage subsystem to install. Even then you can make a change to allow for IDE/SATA. But you know as well as I do that one should not be trying to offer a enterpise solution without SCSI/SAS anyways.
I have seen the free version boot up and run on some really old hardware. It's only complaint is the storage sub. And the free version does allow for central management. Migrating (Vmotion) does require VI and at least ESX server.