Alright so quick update.
After a day of trying, I finally got a successful install of win 10.
Downloaded Doom and even woke up early to play before work.
A half hour in it crashed and restarted.
At work I realized I hadn't updated drivers. So I did that when I returned home. Played maybe and hour before my comp shut down.
Now... after letting it "cool" down. the (new) power button triggered no response. I flicked the PSU off and on then tried again.
Everything clicked on for about a second before turning back off.
If something's wrong I don't want to exacerbate it. But I'm at a total loss what it could be.