Hi pattosk,
Welcome to the community. Yes it is possible to install Windows Server 2012 Essentials can be installed on a VM on ESXI. Windows Server 2012 is fully supported for ESXi 5.1 and ESXi 5.0 U1 and ESXi 5.0 U2. If you are running any of these versions you should be good to go. Upgrading ESXi does not have anything to do with Windows guest licensing. So, no you will not lose the license after upgrading ESXi. And yes you can upgrade the ESXi in future without worry.