I'm not convinced that a pile of old HDDs is a good fit for your homeserver.
Many small disks will consume more energy than fewer large disks. Currently, the best capacity per price seems to be in the 20TB segment. A similar argument can be made for noise.
The HDDs you have might not be perfect for 20/7 usage. I personally would recommend using disks that are made for continuous usage.
Start with what you have, but if you outgrow your setup, buy proper hardware. And make backups.