HP z820/z620/z420 and Dell T7600/T5600 are great used workstations that support the Xeon E5-26xx V1 series processors at very reasonable prices on eBay. Workstations are $300-$700 (maybe add CPU, RAM, HD, graphics card) shipped. I've bought a z820 and z620 at $420 & $499 respectively. My machines were partially usable missing only a second CPU & graphics card in the z820. z620 was fully usable with dual E5-2620s. I added a pair of E5-2660s, 128GB of RAM, and a second CPU cooler+fan to the z820 for $500 shipped and swapped the graphics card. So for $920, I got a 32 threads and 128GB beast z820 (probably would have cost $20K new) and a E5-2640 and 16GB of RAM to sell. So easy to work with too with their tool-less design.
It's a great 4K video/photo editing & OpenShift lab machine. Literally 300 chrome tabs and it hasn't cracked past 16GB of RAM usage in RHEL. Just need to spin up some VMs now.
I am using the Fractal Design Define XL R2 (what a name) for my Supermicro dual CPU mobo, I drilled one extra mounting hole and the mobo mounted just fine.
To the extent that dual socket LGA2011 ATX/LLB boards moved up in price to a point where you could pay three or four times more for a suitable mobo than for sixteen cores of CPU.
It's a great 4K video/photo editing & OpenShift lab machine. Literally 300 chrome tabs and it hasn't cracked past 16GB of RAM usage in RHEL. Just need to spin up some VMs now.