Given an ESXi free host, is there any way to encrypt the virtual disks of the guest VMs?
If so, what is the minimum ESXi version?
Are there other prerequisites for the host and/or the guests?
VM encryption requires vSphere Enterprise Plus or Platinum: https://www.vmware.com/content/dam/digitalmarketing/vmware/en/pdf/products/vsphere/vmware-vsphere-vs...
You can use Truecrypt or similar tools - completely independant of the vSphere version.
VM Encryption using the same name vSphere feature is not possible with the license of VMware ESXi. You need at least vSphere Enterprise Plus and some additional stuff, like a Key Management Server.
But you can use disk encryption inside your VM, like Microsoft Bitlocker.