You could use some containerized mail server like Mailcow. They’re pretty alright to set up and should work fine for low volume. At least in my experience. Unless you don’t want to deal with mail yourself, then you should maybe consider a paid service. But I don’t have any experience with those.

I think what you’re trying to do is called a VPN. Set up a VPN that tunnels all the torrent traffic to and from the virtual server.

Most people use either Matrix or XMPP. Both work.

There is a nice overview of chat protocols here: https://www.messenger-matrix.de/

I mostly use matrix as of today. I think it’s alright. It’s a bit difficult to explain encryption and device verification to other people… I think that could be designed better. But apart from that it works very well. So does XMPP which I’ve used before that. Have a look at the messenger matrix and all the options before deciding on an ecosystem. I’d take one of the friends and do some evaluation before dragging the whole group in. You can do that with some pre-existing servers before learning how to host the server part.

And btw: With most of them you can just use some public servers. You should do that unless you’re willing to put in the effort to maintain an own server. That’d give you complete control over the infrastructure… But it’s also a liability to maintain a server, do the updates etc for a group of friends and maybe years to come… End to end encryption will keep the content of your messages private, anyways. (If you use it.)

Someone linked a list of Mini-PCs here: https://lemmy.world/post/19837516

I think the N100 sounds good. But I can’t comment on buying a cheap chinesium one versus a refurbished Fujitsu/Lenovo or an Intel NUC.

For AI training? Or why would someone specifically pay attention to watermarks? I believe there are curated datasets out there. And watermark detectors to weed out watermarked pictures from a pile of data. I don’t think the general public sorts by presence of watermarks…
And I’m not sure about games and webcomics. They all have some logo somewhere, because they’ve been made by someone. If you don’t want that, you’re looking for white-label products. I think in the realm of privacy, and those product types, that’s a small to non-existent niche.