I was selling a kit of DDR4 RAM and was offered this HP Z400 as a part trade. It was valued at 50$ for the trade, which was greatly in my favor. It came with a Quadcore Xeon CPU and a Quadro GPU that I ended up selling together for 80$. I replaced the 4C 8T CPU with a 22$ 6C 12T X5660 I got on Ebay and threw in a GTX 1060 3GB that I had been trying to sell.
I was really happy with the completed build and I almost kept it because it was better than the gaming laptop I had at the time (i5 7400 - 1050Ti 4G), but I planned on building a custom system soon and decided to list it for 400$. If I remember correctly I ended up selling it for 360$.