Used corporate mini-pcs make excellent home servers if you don't need too much storage (typically they support two SSDs), and want low power consumption & noise and small form factor. Right now ones with 7th or 8th gen i5 are a pretty good deal, often around 50-120 USD on ebay, a bit more in the EU. Idle power cosumption is about 10W. If you want to play around with virtualization, a higher-end model with Intel NIC is recommended due to some Linux driver issues (at least with Proxmox).

https://www.servethehome.com/introducing-project-tinyminimic...

Corporate thin clients with Pentium J5005 or similar also make decent raspberry pi replacements, usually just not as good price-power ratio as proper minipc. Certain models have pcie slot, which makes them ideal DIY routers when fitted with pcie NIC & pfsense/opnsense as OS. If you're using a consumer router, such project can make lots of sense.

For NAS, I recommend building your own. Personally I use Asrock J5040 mini-itx board with Node 304 case and Unraid as OS, houses 4x 3.5" drives, so far happy with it. For maximum reliability you might want a mothebroard that supports EEC memory though.