Hello.
The first thing is to validate that the physical servers support the version of VMware vShpere chosen, I attach link
http://partnerweb.vmware.com/comp_guide2/search.php
If the server is a recognized brand is easier to look for compatibility with VMware vSphere,, such as DELL, HPE, Lenovo and more.
The manufacturers also offer custom images (ISO) for VMware vSphere installation, which are recommended to install VMware on your servers.
Basically you need two types of licenses
VMware vSphere licenses per Processor (ESXi)
a vCenter Server license
A physical server with 2 processors will require 2 VMware vSphere licenses.
In your particular case it would be 8 VMware vSphere Lic 1 Proc licenses and a vCenter Server license Standard.
There are license kits that include both VMware vSphere and vCenter Server licenses,
Along with the licenses includes a software contract and subscription for 1 or 3 years.
The licenses currently sold are for version 7, but by registering licenses in Customer Connect (My VMware) you can downgrade your licenses to get licenses in version 6.
Attached is a link to the details and features of the licenses.
https://www.vmware.com/content/dam/digitalmarketing/vmware/en/pdf/products/vsphere/vmware-vsphere-pricing-whitepaper.pdf