Part 5: A light at the end of the tunnel...
... for the first modification, the internal PSU, that is. Some progress being made, soon it will all come together.
The last cable to be prepared was the PDCB input, which was straightforward. Hooking it all up outside the case and doing a first test to see if the PSU/PDCB combo works as designed. I decided to leave the inlet and switch inside the case - as removing the switch from the flimsy case seems impossible once it is in place.
Switch and inlet do their job, same goes for the PSU and PDCB, as the PSU tester shows stable voltages. Heat was of course no issue, as there wasn't any relevant current being drawn.
Test fitting the components inside the case also was successful, all parts play together nicely. LEDs were glued to the front panel and tested again, but at the moment I am a little bit stuck trying to get the new power button to operate correctly. I am still having issues finding the right way to add some padding, so the button will indeed press the tiny switch reliably.
Thank you for your comments, and you're absolutely right about the spray paint. Here is a little behind-the-scenes view of the raw cutouts, now you know why I tried to hide it from the public...
Thank you! I had indeed been wondering how you managed to cut the holes for the inlet so perfectly 
I am actually thinking about ordering another case, they are rather cheap now on AliExpress. I found a seller where the case costs less than 40 Euros, including VAT (which AliExpress collects for shipments to Austria since July 1st) and shipping.
As for the screen, I am not happy with those 2 hinges I was considering. Using them won't allow to prop up the screen at an angle, there would need to be another mechanism for that. And I think it's an absolute must to use the screen at an angle. While the horizontal viewing angle is great, the vertical is not. There's only a few degrees of tolerance, before the image looks all washed out. So more thinking and googling (or rather binging) needed in the days ahead on how to mount the screen and enabling it to be lifted up and fixed at any arbitrary angle.
Next steps:
case modification for inlet and switch: donewait for a replacement case to arrive because I ruined the case in step 1 
: surprisingly enough this wasn't necessarypreparing all the necessary cables in the correct length (and waiting for connectors to arrive): donetesting PSU and PDCB with the PSU tester: doneif all goes well, hooking it all up: PSU, LEDs, PDCB, power switch and modified USB3 ports: done, except for power switch and USB3 ports, which will just be glued to the case- testing the computer for performance, thermals and airflow
- lots of thinking,
googling binging and learning to tackle the monitor case lid...: halfway done, I hope
