Hi and welcome to the communities,
you are right on the 5.x licenses. You just need to enter the serials into vmhost1 and 2 and they are capable of essentials plus features then.
Btw there is an enhanced vMotion process, that can be triggered using the vSphere web client since vSphere 5.1.
You can do a combined vMotion and Storage vMotion, so you can shift your VMs to another host, even with only local storage.
It will take some time, because the whole VM needs to be copied over the network, but for an upgrade scenario, this will help ( for vmhost2 at least )
I didn't quite get your NFS plan. Do you want to install a VM on one host that exports a NFS share, that is used by all hosts as datastore?
I woudln't do this, as a single VM or Host failure would bring down your entire environment.
Regards
Tim