I just installed Server 2012 onto the gen8 no problem. All I did was to copy the iso to a usb stick with rufus 2.2, then pluged the usb stick into the port on the front of the Gen8.
Then I loaded up the Drive config utility, set my single drive as a raid0 and made sure it was set to be allowed bootable. restart and then the install started automatically.
I made sure I copied the HP drivers for the SATA controller onto the disk. I just put them into a directory on the install usb partition and pointed the install to them when needed and it found the disk and that was it.
I want windows 8.1 as I don't want to destroy the data on my drives and install on a USB drive that's in the machine is possible. Server apparently wipes all the drives. I may need to get a ssd drive as a spare drive but don't fancy hacking any power cables and joining them to fit one.