The limitation of Essentials Plus you should consider is the 3 hosts limit. It doesn't really matter if it's already in production or not.
Also, make sure that the hosts are capable of running the desktops. If vSphere is already in place, you should make sure you have enough resources for VDI.
Make also sure if the Horizon View version you are about to install is supported by your vSphere version. You might have to install View 5.2! Also, take a close look at system requirements for Horizon view (https://pubs.vmware.com/view-52/topic/com.vmware.ICbase/PDF/horizon-view-52-installation.pdf).