Purchase all the supported hardware and Assembled to work with latest ESXi.
Go to VMware HCL and search by component wise and price. Once all are supported then you can assembled.
It is an authorized VMware partner PSP as well.
For what purpose do you want to buy the hardware? Home lab, production, ...?
There are a lot of shops selling refurbished server hardware on Ebay for example, but there are some things to consider. If you get a used/refurbished HP Proliant which does not have a support contract with HP you are going to have trouble to download latest BIOS and/or Proliant Service Packs (SPP).
I would prefer a vendor having customized installation ISOs for VMWare, which I think only DELL, HP and IBM have (maybe even Fujitsu, but not fully sure about this).
Check the specs of your desired server model reagrding storage controller and built in NICs if they are supported by either native VMWare or by the server vendor.
In general I would get more RAM, 8GB are not enough as ESXi takes about 4GB of them leaving 4GB available for VMs.
I would search for a late generation HP or Dell rackmount server that is still on the VMware HCL.
Since 1U or 2U rackmount servers do not attract the gamer crowd, there is incredible value on servers just one generation old and still fully supported. I mean just a couple hundred bucks gets you a great lab server.
For convenience, look for 1U servers like an HP DL360 or Dell R620