No need purchase anything: Evaluation license is good enough to start with.

Refer:Free Virtualization with VMware vSphere Hypervisor (ESXi) | VMware India

Welcome to the Community - no you will not need to buy anything but you will need to register with VMware to get the associated free license -

With the free license you can use the provided vSphere Client. Once you install vSphere on your host do the following:

1. From a Windows machine, open a Web browser.
2. Enter the URL or IP address for the host. For example, http://exampleserver.example.com or http://xxx.xxx.xxx.xxx.
3. Click Download the vSphere Client under Getting Started.
4. Click Save to download the vSphere Client installer.

Once installed connect directly to your host using the IP or hostname.

**At this time there is no Web Client for the free version. If you plan to use this for an extended amount of time DO NOT upgrade your VM hardware to version 10, you won't be able to edit them with the vSphere Client. Here's some more info: Re: How can I manage the free hypervisor 5.5 without the use of paid vCenter?

Plz note that with free hypervisor you have only read access to APIs. You can manage VMs by using corresponding VI client. 

Please refer this post, it is worth to go through :Re: How can I manage the free hypervisor 5.5 without the use of paid vCenter?