As of vSphere 5.5 all the core and memory limitations have been removed, and licensing is simply based on the number of physical processors/sockets (see http://kb.vmware.com/kb/2064117).
All you need to make sure is that the license key you are going to assign has at least the required CPU capacity. You can merge license keys for the same version/edition even with with different SnS dates. However, this will not change SnS and the end-dates will still show up ion the MyVMware portal.
vCenter Server is optional and licensed separately, and will be required if you want to manage the host in a cluster, or use the additional features (e.g. cloning, live migration, ...)
Thx André for quick and helpful reply