In that case, I can recommend minicomputer’s like HP EliteDesk G2 800 Mini. You can get them with a variety of intel CPUs, they can take up to 32GB RAM, they have slot for M.2 disks and a regular 2.5" SSD – and they hardly use any power when idle, between 5 to 10 watts, depending on the CPU and CPU governor settings. They are sold used for ~€50 and if you buy newer generations you’ll get even more umpfh for a bit more cash.

In other words, very competetive with the Pi’s, only more available, cheaper and about the same power consumption!