Welcome to the Community,
you can get a free license for ESXi 4.x as well as 5.x. What both of them have in common is that you can run them only as stand alone hosts. They cannot be added to a vCenter Server instance. The main difference - besides the improvements in the newer version - is when it comes to licensed features. With ESXi 4.x there's basically not limit regarding memory (256GB), but there is a limit of 6 cores per physical CPU. With ESXi this was modified and you now have no more limits regarding CPUs and cores, but you are limited to 32GB RAM per host.
In your case, an inexpensive alternative to the free edition could be a vSphere Essentials license which includes 1 instance of vCenter Server as well as CPU licenses for up to 3 hosts with 2 processors each. With this edition, the vRAM entitlement (sum of memory for powered on VMs on all hosts) is 196 GB. In addition to this, with this license you'd be able to use vCenter Server features as well as e.g. 3rd party backup applications.
André