Can we use that one and what is the disadvantage of using this version?
As you have mentioned you can have only maximim of 32 GB physical RAM, others include
Read-only access by CLI, NO SNMP support,
Due to API limitation cannot use all backup software to backup the VM's.. Limited to ghettoVCB or Trilead VM Explorer
If you have decided to go for the paid version then you have to check the feature which you are interested , check the feature avilable with the each license
http://www.vmware.com/products/datacenter-virtualization/vsphere/compare-kits.html
Is there no limit for the number of Vms on the host or the maximum memory in the hosts?
There is no limit as such but you to check the maximim guide to make sure that you dont cross that level as this has been tested value by VMware.
It looks like the Essentials kit is something for us.
It's a lot cheaper than the vSphere standard version, why?
The differernce in the cost is becase of the feature in differernt version of license