I highly recommend the BYO route, especially if you have part of a suitable machine already lying around. Just about any x86 based system that can take 16GB+ of memory will be a good candidate. ECC is recommended but many people, myself included, have run many stable years without ECC. Load the latest Ubuntu up and give this [1] ars how-to article a read (keeping in mind it is several years old now). Power efficiency will mostly hinge on the cpu and how many hard drives you keep spinning. To scale past the the number of SATA ports you have on a motherboard, add an LSI HBA card like the 9300-8E or 9207-8E.
[1] https://arstechnica.com/information-technology/2014/02/ars-w...