Get a N100/N150 system with 12GB+ RAM for ~150 €/$. Alternatively check for one with replaceable RAM.
To get experience with Linux you can install VirtualBox on Windows and set up some Linux virtual machines. It’s easier than most people think.
Because at least when you use their private routing feature messages go: sender -> custom sender’s server -> custom receiver’s server -> receiver
So unless sender and receiver are configured to use servers under your control, there will be at least one foreign server involved.
To be fair I’m not sure how that works, when you don’t use their private routing feature.
If you like SimpleX, you can host a server and configure your clients to use that. You don’t use only your server though, so I don’t know if this will satisfy your requirements.
I’m pretty sure GOG releases checksums so you can check against those whether you have the right files.
Then you’ve never used the German Kleinanzeigen.de.
Okay, so you’re saying this will never be broadly used. Got it.
I gave the demo a quick try and I really like it. Simple and to the point, no unnecessary fluff. I might set this up at home.
Thank you for your work!
Edit: One thing, the demo exports notes as PDF with white text on white background. I assume this is a mismatch with the selected theme.