First I discovered that "full vCenter" cannot manage Essentials hosts which is grating. Then in reading around editions I discovered the presence of a vCPU (Virtual SMP) limit on guests, but we need to provision one at 24 vCores, which would put it at Enterprise for 5.1. I pinged our hardware vendor who also acts as our VMware vendor and said "figure it out for me" and the initial answer I got was Enterprise Plus, but after pressing the matter the rep said "I'll double check"
Fortunately, while waiting for the reply, I found VMware KB 2001113 which showed that yes, there was an 8-vCPU limit on editions through Standard in 5.1, but there's a "See note" under 5.5. The "note" is KB 2064117 which says in respect of "all paid editions":
There are no restrictions on the number of vCPUs per virtual machine. You can configure up to the maximum number of vCPUs per virtual machine as specified in the vSphere 5.5 Configuration Maximums Guide.The free hypervisor remains limited to 8 vCPU per guest (but the 32Gb/host limit is gone).
So if you don't have a reason you need to stick at 5.1, even Essentials is sufficient to run even what is for us a ridiculously large 24 vCPU guest. Now I wish I hadn't spent so much time on an SCVMM/HyperV plan B...