What do I need to buy to be able to run ESX on one or two hosts and use vCenter and PowerCLI to administer the hosts? Does the "Essentials" kit do it? What licence do I need? I see that I can download PowerCLI, but will it work with just any ESX or vSphere installation?
Unfortunately VMware decided to lock up the ESXi automation API accessed by PowerCLI and other tools, so the API is read-only on ESXi hosts running with the free "vSphere Hypervisor" license. You can still perform "read" operations such as Get-VM or Get-VMHost, but any "writes" with Set-* Add-* New-* Remove-* etc cmdlets will not work with the free license.
Any official license is sufficient to lift this limitation, so you can get the cheap essentials kit. Of course you are still bound by the general feature limitations of that license then though.
The other not very great option would be you re-build your lab every 60 days with the evaluation licenses.