2011 Mac Mini - Boot Issues..

[Virus__]

So I ended up with a 2011 Mac Mini "Server" Core i7 2.0GHz, 750GB HDD, 128GB SSD, 16GB RAM.

I was having some OSX issues out of the box, like it not recognising the Admin password in some parts of system preferences etc, small things.. Then it got stuck in an endless boot loop, which was frustrating. Managed to reset the PRAM & NVRAM & then nothing.

Managed to get it into recovery mode so I could reinstalled OSX, it was offering Lion, that was fine, then it gave me the error "not all components could be downloaded" okay, so i'll just create a bootable copy of OSX like I have done in the past. This is where I run into issues, so the latest version of OSX I can run on this is High Sierra, no problems, i'll just get a USB drive & get the installer from Apple, nope, it doesn't exist in the Mac AppStore any more.

Okay, my next step was to get the .DMG from Apple of High Sierra & create a bootable via terminal, that didn't work either. Now I have managed to create a bootabale USB drive from inside Windows, but whilst it is recognised on my MacBook Air, it's not recognised by the Mac Mini when I try & use it to boot from at start up by holding down option, and I can't get it to boot from it on my MacBook Air so I can install it on an external SSD, i've run out of ideas on how to get OSX back on this Mac Mini.

I've run out of ideas. All I want to do is install High Sierra onto this damn thing so I can use it, it shouldn't be difficult but it turning out to be...