In my experience, the country/language-specific TLD weightings of the past are no longer as important as they once were. The prevalence of vanity domains like "bit.ly" (Libya) have made it more complicated. I am confident if your Money Site is a .COM in German and you've used all the HTML markup to declare it is German there will be little difference to the same site being hosted on a .DE
In terms of building a PBN, the best way to start is research! Check existing authority sites and look to emulate their backlink profile, here is a quick Moz of
www.spiegel.de
Only 16% of their backlinks are from .DE
On the other end of the scale here is another report for a random computer-retailer in Germany where 61% of the backlinks are coming from .DE
So crudely piecing this together with 2 data points. If I were Google and just discovered your Money Site and it had between 15-60% of backlinks coming from .DE it would appear "natural" because I see that pattern all the time. If your site had 100% .DE links it would be suspicious/unnatural.
Summary:
* Put your money site on a .DE domain, cant hurt
* Aim to build 33% of your backlinks from .DE domains (either existing sites you can create/buy links from, or your own PBN domains)
* Build the remaining 66% backlinks with a mix of .COM/.ORG/.NET and some local/regional ones .AT/.FR/.BE/.NL etc.
Now you only have 1/3rd of the domains to worry about registration details, because the rest of your PBN can be on other TLD's
