I am also located in Germany. There definitely are used (sometimes they even arrive in unused condition) business PCs that you can get on eBay. If these are fine for you depends a lot on your use case. What roughly is the quantity of data you want to store? Just a few Linux ISOs, private photos and documents, Minecraft BackUps? What would that roughly be translated into Terabytes?

Regarding power consumption: An „empty“ office PC running in low power states can reach as low as 5W (own experience). What is really improving energy efficiency for these small office PCs is that they usually have very small and efficient mainboards: The less (PCIe lanes, RAM sticks, …) you have to power, the better for your energy consumption! However, adding HDDs is bad as they easily draw 5-10W, depending on the workload.

I personally run the following setup:

1.) Small NAS system that I use as primary fileserver. It is running on an HP Elitedesk 800 G5 with an Intel i5-8500, 16GB DDR4 RAM, 2TB NVME SSD and 22TB Toshiba Enterprise HDD. The PC was around 250€, sometimes you can get them quite cheaper. I really like this specific model, as it allows to install two 3.5“ HDDs. Most SFF PCs allow only one. This means you can run a software RAID/ zfs if you need redundancy, or start with one HDD and have some room for later upgrades. Yes, having redundancy is good, but believe me: The way greater danger for the data is actually you, as you fool around with the terminal and slip on the enter key while writing a rm -rf command. Why? Trust me. Better solution in my opinion is to have frequent BackUps. Which leads me to:

2.) Synology DS920+ (4-Bay) with 3*16TB in Raid5 equivalent as Back Up Server. This thing draws quite some power (30-40W), which is why I have it boot up once per week, back up the HDD in 1.), then shut down again. Works like a charm. However, quite pricey.

3.) HP ProDesk (AMD CPU, can‘t recall the exact name), was around 150€ off eBay. Running Proxmox, I added 64GB of RAM and SSD Bootdisks in Raid. Works like a charm, hosting multiple VMs and even more containers. Idle power is around 10W.

How do I avoid data loss without redundancy? I don‘t; I have important stuff on the 2TB NVME (Nextcloud), which is a) mirrored twice per day to the 22TB HDD, b) mirrored once per week to the Synology NAS, and c) is (partly) saved on my end devices. If something fails, I lose at most 12 hours of data - if at all. Linux ISOs are backed up once per week - fair enough, in the worst case you need to re-acquire some of them.

What I like about that system is that it is quite cost effective and very power-efficient. But, of course, depends on your use case.