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.