Q1.) The Essentials Kits are limited to 3 jots with 2 processors/sockets each. With vSphere 4 there's a limit of 6 cores per socket, this limit has been removed with vSphere 5.
Q2.) You don't necessarily have to upgrade. If you current hardware does not support ESXi 5 you can downgrade your licenses to version 4 in the My VMware portal and upgrade them again once you purchase new hardware.
Q3.) There's no special evaluation for Essentials. All the editions share the same binaries and it's only the license key which locks or unlocks features. So you can download the 60-day vSphere Evaluation to test it.
Please don't mind me asking, but what's the reason behind purchasing the Essentials Plus Kit? Do you need it to unlock the API, i.e. use VM based backup applications? With only a single Host I don't see a reason for the Essentials Plus Kit anyway, the Essentials Kit should do!?
André