Xeon E5-2699v3...
The Xeon E5-2699 v3 was the best balance I could find between core count, frequency, and price for this build. It’s an 18-core/36-thread chip, and with MadMax I’m able to push 34 threads while leaving a little headroom—just be ready to deal with 145W worth of heat in a 1U chassis.
You'll be taken to Ebay to complete your purchase.