you can buy the broadcomm card that's in the macbook air from 2011 from ebay and an adapter for it that connects to m.2 wifi. The screw for m.2 wifi aluminium shield can be unscrewed and you can change the card (i believe) but please check the board and upload pictures if you do this.
from here in southern California, i'm not sure what "Greats from Austria" means. maybe s/he means ("s/he" denotes a problem with English itself; choosing a gendered pronoun when the gender is uncertain) "Greetings from Austria"? anyhow keep posting, we'll figure it out.
i can't imagine building a computer with instructions in a language i wasn't familiar with, but it looks like it's commonly done.
ErikKranzinger quote: When i have all Parts I will write all my Problems and Work Info down here on this Post. end quote.
keep in mind that this forum is for discussion about "buying advice" and so is focussed on hardware compatibility during the buying phase and other forums on this site would be more appropriate for "getting the computer working".
from a new person (inexperienced) point of view, there might be some fear that the parts are incompatible, however, if one does a good job here with selecting the parts, the next phase might go smoother AND those experts in the other forums are probably good with problems that arise from hardware compatibility issues.
once you have all your parts, then look at the +- 13 forums here https://www.tonymacx86.com/categories/installation.128/ to post in and after that, look through all the forums on this site. there are lots of specific categories where you might get better help than posting here.
be sure to keep your profile hardware up to date so they can understand what you are dealing with. i am new to this forum, but should you change your profile to what you put in your post? repeating this question to everyone here: should he (assuming he is male [Erik]) change his profile hardware?