Your currently stated requirements would be fulfilled by anything with a general-purpose CPU made in the last decade and 2-4GB RAM. You could use almost literally anything that looks like a computer and isn't ancient.

You're going to need to go into more detail to get any advice worth following here.

What home servers differ most in is storage capacity, compute power and of course cost.

• Do you plan on running any services that require significant compute power?
• How much storage do you need?
• How much do you want it to cost to purchase?
• How much do you want it to cost to running?

Most home server services aren't very heavy. I have like 8 of them running on my home server and it idles with next to no CPU utilisation.

For me, I can only see myself needing ~dozens of TiB and don't forsee needing any services that require significant compute.

My home server is an 4 core 2.2GHz Intel J4105 single-board computer (mATX) in a super cheap small PC tower case that has space for a handful of hard drives. I'd estimate something on this order is more than enough for 90% of people's home server needs. Unless you have specific needs where you know it'll need significant compute power, it's likely enough for you too.

It needs about 10-20W at idle which is about 30-60€ per year in energy costs.

I've already seen pre-built NAS with fancy hot-swap bays recommended here (without even asking what you even need of it, great). I think those are generally a waste of money because you easily can build a low-power PC for super cheap yourself and you don't need to swap drives all that often in practice. The 1-2 times per decade where you actually need to do anything to your hard drives, you can open a panel, unplug two cables and unscrew 4 screws; it's not that hard.

Someone will likely also recommend buying some old server but those are loud and draw so much power that you could buy multiple low power PCs every year for the electricity cost alone. Oh and did I mention they're loud?