If I remember correctly, most drives either:
1. Fail in the first X amount of time
2. Fail towards the end of their rated lifespan
So buying used drives doesn't seem like the worst idea to me. You've already filtered out the drivers that would fail early.
Disclaimer: I have no idea what I'm talking about
Over in hardware-land we call this "the bathtub curve".
we don't have perfect metrics here but this seems to match our experience; a lot of failures happened shortly after install before the bulk of the data download onto the heap, so actual data loss is lower than hardware failure rates
Where did you source them? I've thought about buying HDDs from a vendor like serverpartdeals.com but was unsure how reliable the drives would be.