I bought a Dell Optiplex 9020 recently. i7-4790, 8GB of memory, 500GB hard drive, proprietary power, but a standard MicroATX form factor motherboard.
Transplanted that into a full ATX tower (what was available), built my own power supply adapter 24-pin to 8-pin. Actually used a dual 6-pin to 8-pin PCIe adapter for the guts of it, worked out perfectly all they had done to key it was add a line of plastic I was able to cut out. Had to permanently short standby to ground to get it to turn on, the front power switch was a little too complicated to reverse engineer in an afternoon. Had to re-wire proprietary 5-pin fans. And I went ahead and installed larger cooler. Which took a little doing as Dell uses a custom backplate/socket. Luckily...