When I build my NAS/server last year, I bought a used Dell Optiplex from 2013 on eBay for $50. I tossed in an old SSD I had laying around, and squeezed in 42 TB worth of HDD drives.

The only real downsides of doing it this way are

• No realistic way of upgrading hardware
• Limited space for internal drives
• No hardware transcoding abilities out of the box
• More power consumption than buying something newer