HP Z2 G4 Workstation
$150 + shipping
Intel i5-8500 CPU
8 GB DDR4 RAM
512 GB SSD with Windows 10 pre-installed
This would make a decent NK 6.0 build. It only has 2 slots for 3.5" HDD but you could fit 2 or 3 more in the 5 1/8" bays with a 3d printed or off the shelf bracket.
PM me with your address or zip code for a shipping quote.