i really like this software-centric approach, but I am a bit stuck when it comes to the hardware. Are there some sources around servers that don't suck up too much energy and are fairly quiet? What CPU is a good all-round solution?
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If you need more power: I had success with HP ProDesk Mini (or any other one-litre PC), you can get these second hand from like $150 and extend RAM and SSDs however you like. You can even pick processor / generation to fit your needs best. These can have consumption from like 30W if I'm not mistaken.
I have no experience with real and expensive server hardware, but most people don't need that for a homelab.
Wut? For many, a Raspberry Pi with 1 GB RAM and the regular sdcard can be enough, you really don't need to go fancy if you don't want to run anything particularly heavy. Or if it's cpu-intensive then you might need the newest Pi or something even beefier but still only the lowest RAM and smallest/slowest storage options (like for WordPress). As you say, it depends on needs
I always recommend using an old laptop to start out with because you've already got it anyway and it's already low power yet very powerful: if it can run graphical software from 2020 then it'll be fine as server until 2030 for anything standard like a web server (with half a dozen websites and databases, such as a link shortener, some data explorers, and my personal site), torrent box, VPN server, mail server, git server, IRC bouncer, Windows VM for some special software, chat bot, etc. all at once. At least, that's what I currently run on my 2012 laptop and the thing is idle nearly the whole time. Other advantages of a laptop include a built-in KVM console and UPS, at least while you still trust the old battery (one should detach and recycle that component after some years)