A quick introduction; I'm the creator of this (fictional) Dutch route and it's very nice to see some people over here are interested in driving around the Netherlands
I'm not particularly skilled at written German so I will reply in English instead; altough i can read German just fine.
The catenary used within this route may be found here; please note that if you previously installed this package (required for another Dutch route for example); you update it to its latest version; or you may still miss some things at particular sections
Regarding the windmills it depends if you mean the classic ones made out of brick or the modern ones that can be seen all over europe these days:
The older ones: Link (you only need the assets; not the route itself)
The modern ones: Link -->NS Station Objects & scenery
Just like the catenary be sure to also install these if you've done so before because all these packs are regularly being updated
If there are any other questions let me know