I am running some services like Jellyfin, Radarr, QBittorrent, Jellyseerr and some others on my Raspberry Pi 4. The problem is that it is already struggling to run those, since it has only 2GB of RAM. I wish it was possible to do a RAM upgrade to the Raspberry Pi but the RAM is soldered to the motherboard. I don't want to buy another Raspberry Pi with more RAM because they are quite expensive and I don't want to have two of them. So can you recommend something for around or under 100€?
I don't understand the fascination of other commenters with mini-PCs, as the mini-ness was mentioned nowhere in the OP.
any used and decomissioned old office PC, any i5/i7 is way more powerful than you'll need for that setup. you get everything you need right in the box and you can cram it full with cheap RAM and hard disks. you get to repurpose something that's useless as a desktop workstation and not buy more future e-waste.
yes, the mini-PCs and the Rpis are more power efficient, but the operating costs of a $30-50 PC don't come close to the price of buying one of these mini-things, not to mention - figuring out how to run large hard disks with it.
I have acknowledged that they're that, but that's not what OP asked for - they asked for a cheap setup (which the minis ain't) and they intend to run a servarr instance, which implies large storage and those are both difficult and not cheap to cram into said minis.
It depends on your needs. I have minis that cost <100$ and have others that cost 500$. My cheapest mini has currently 3TB of backups of my personal things, so it serves my needs very cheaply. I don't need a GPU so it keeps the costs down.
I agree that desktop/ATX tower PCs are the most useful form factor, you can stuff all your old junk hardware in there and offer it a second life without much investment.
However with current electricity prices buying more power efficient hardware can be a better medium-term investment. 1kWh bills at 0.2516€ currently where I'm at (~EU average price), assuming an average power consumption of 50W this gives you (50×24×365)/1000×0.2516=110€/year. At this rate a 200€ investment in hardware would pay for itself in 2-3 years.
Buying a <100€ setup is not worth it for general purpose servers in my opinion, it will either be underpowered or power hungry.
My current solution is to to run all my services in KVM (libvirt) VMs on my beefy desktop computer which is already on most of the time anyway. Best of both worlds.
If I had to redo everything I would probably buy a NUC/mini-PC with a good CPU, 64GB RAM and low power consumption, stash a single huge SSD in there, migrate my VMs there and call it a day. But this is not a cheap setup.