So to summarise, if I buy the (doesn't the clipboard work in here) "VMware vSphere Essentials Kit", I can download PowerCLI (or does it come on the CD/DVD/image?) and use it with a writable API?
1. You can already download PowerCLI for free. You have to register, in worst case maybe for a vSphere eval, I'm not sure; but there is no eval time restriction for PowerCLI or anything.
2. You can "use" (as in run cmdlets on the system where you installed PowerCLI) for free.
It's just that if you run those cmdlets against a target ESXi host which runs with the free ESXi license, this host will reject all write operations because the API is read-only with the free license.