I have a secondhand Dell Poweredge T320, with a single Xeon E5-2428L CPU, its server-class, but in a tower format (the T in T320), so it's the best of both worlds, and the CPU is low power (the L in the name). Cost me 400€ with 24GB RAM and some disks.

It's been running for over 6 years I think. It's only noisy for a couple minutes when starting up, then it's pretty quiet.