Grey screen when booting from original disk, doing Internet recovery or booting from High Sierra boot disk
The flashing question mark means that it could not find a volume to boot from. This indicates either that the volume is corrupt, disk not working or an error in the SATA controller that talks to the disk (or all of the above). The grey screen I believe comes from the stuck storage device enumeration right before the OS X Utilities window should show.

I've encountered the exact same behaviour on a MacBook Pro of similar age and it in my case it turned out to be a failure on the internal hard drive controller. I'd suggest you open it up and disconnect the hard drive, then try starting from Internet Recovery, the diagnostics and running from an external storage (such as live CD or USB stick). It may be that only your hard drive failed so a drive replacement could be a complete fix.

It is quite easy to open the computer and temporarily disconnect the drive, see the detailed procedure at iFixit