So, let's keep it short: for the time being, I've bought the second-hand Lindemann Musicbook 55; I think, for a reasonable price.
For me, the question now remains which streamer it will be. I have ordered the Node N130 and the Inklang HD10.
So there are the following options:
1. Inklang Streamer/DAC --> Power Amp (disadvantage: no Roon Endpoint).
2. Node N130 --> Power Amp (you read not-so-great things about the internal DAC of the Node? Therefore, cheapest solution)
3. Node N130 --> ext. DAC (e.g. used Denafrips Ares or RME ADI) --> Power Amp
The price for 1. and 3. is around 2k each, solution 2. would be 1300€.
This hifi stuff really invites you to "run in circles".