I finally received most of my ordered parts and now getting started.
First of all I want to express my thanks to the users that helped me very much:
[mention="wermy"]wermy[/mention] for setting up this awesome forum and tutorials,
[mention="Fleder"]Fleder[/mention] for gathering those screen-hacking tutorial (couldn't have done it without it),
[mention="chiz"]chiz[/mention] for the awesome shoulder-buttons build idea,
[mention="dominator"]dominator[/mention] for making and selling that button PCB.
First I salvaged my old DMG from the attic. The battery cover is missing, thats a bummer.
[spoiler="Gameboy DMG without battery cover plate"] [/spoiler]
I ordered an other broken DMG from ebay, but I wont need that cover plate until the very end so I could as well get started.
Next I tested my screen (had to hack it first to 5V though) and the raspberry pi zero.
[spoiler="Test setup raspberry pi zero with monitor"] [/spoiler]
Now I drilled and filed the x and y holes, added the button brackets and testmounted the buttons.
They turned out quite fine, the button feedback feels very satisfying

[spoiler="Drilled x,y holes and button testmounting"] [/spoiler]
Then I shaped out the display hole to fit the 3.5 screen. I left a 1-2mm rack for the screen protector.
[spoiler="Shaped out display front"] [/spoiler]
Thats all I could achieved at day one. This is my first modding-project so I'm working really slowly trying not to mess anything up.
But its actually a lot of fun. Hopefully I will soon find some time to continue.
I will definitely keep this thread updated.