I have to imagine that the best NAS build is simply a 6-core or 8-core standard AMD or Intel with a few HBA controllers and maybe 10Gbit SPF+ fiber or something.

"Old server hardware" for $300 is a bit of a variation, in that you're just buying something from 5 years ago so that its cheaper. But if you want to improve power-efficiency, buy a CPU from today rather than an old one.

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IIRC, the "5 year old used market" for servers is particularly good because many datacenters and companies opt for a ~5-year upgrade cycle. That means 5-year-old equipment is always being sold off at incredible rates.

Any 5-year-old server will obviously have all the features you need for a NAS (likely excellent connectivity, expandibility, BMS, physical space, etc. etc.). Just you have to put up with power-efficiency specs of 5 years ago.