VMware ESXi Free vs Paid
VMware has only two ESXi versions, which are, as you've guessed, ESXi Free and ESXi Paid. VMware hypervisor is installed via a single medium and process. You will get the same hypervisor that is the vSphere enterprise quality. It's just that certain features of VMware ESXi will be unavailable to you until you decide to pay for an ESXi license of your choice.
VMware ESXi Free Limitations
Not all the best things in life are free. However, ESXi free can be very helpful when it comes to personal education about virtualization and development, as well as it's decent for testing and disaster recovery. Considering VMware ESXi limitations, ESXi free isn't recommended for production purposes. Since the free version of vSphere ESXi lacks access to support and vCenter Server, it can cause unwanted problems.
That being said, VMware has an ESXi Evaluation license that is meant to help you test the solution. You can use free VMware ESXi to test out how it works on its own or test out other vSphere suite elements, like NSX, VMware vSAN, or vRealize Suite, and others. Such an option provides the chance to see whether you like what vSphere ESXi has to offer, test it all in your lab, and then decide where to go further.
If you're a beginner or a pro that's seeking to deep dive into virtualization even further, ESXi free is great for training and professional development. You can utilize the free VMware hypervisor version to see how it's installed into an ESXi host, and how it's configured, an ESXi works with VMs. On the other hand, you can also use ESXi free to test out Windows and Windows Server versions before you decide to commit to one.
Finally, you can run disaster recovery on your ESXi server by using the free ESXi. Nonetheless, don't get your hopes up too much. VMware ESXi free has a very limited potential when it comes to disaster recovery. If you are a tiny business and you need some modest backup, yes, free ESXi can be used for your disaster recovery. However, using the paid version is advised for regular production and DR.
VMware ESXi Price and License
First of all, VMware ESXi free doesn't have access to vCenter Server or to VMware support. These features are vital for operating your daily ESXi host work, your virtualized environment, disaster recovery, and prevention from downtime. There's only so much the free version of ESXi will offer you in terms of training, testing, and DR. To learn more and get actual use, you'll have to pay the VMware ESXi price.
Managing multiple VMs inside your VMware ESXi virtualization environment without central management is, simply put, difficult and counter-productive. To effectively monitor and manage your ESXi host and VMs, you will have to purchase the ESXi license and the license for vCenter Server. VMware licensing is quite complicated, but, in the end, paying for a bundle is always cheaper than paying per feature.
To ensure constant application uptime and ensure either zero or insignificant level of downtime, you will need to introduce a failover or fault tolerance of some sort. You will need to pay for an ESXi license to receive access to vSphere High Availability, vCenter Server, and a set of other ESXi features to make sure your VMs and other ESXi hosts can continue working even if one of the hosts fails.
Without paying for vCenter Server (VMware's central management suite), patching will overwhelm you and create danger for your virtualization environment. Purchasing a VMware ESXi license means you'll get access to vCenter Server, as well as Update Manager and other tools, which will help you patch all your ESXi hosts and VMs from a central user interface (UI) without any manual intervention or headache.
VMware ESXi Versions
VMware ESXi is a hypervisor with many useful bells and whistles which you need to pay for to use them, but they are ultimately necessary to ensure a smooth and highly performing production virtualization environment. Then again, be sure to check the latest VMware ESXi versions, VMware hardware compatibility lists, and other requirements to make sure you go for the one that best suites your business needs.