Original price doesn't matter, you need to compare it against current new offerings. A drive like that, I'd buy for 8-10€/TB at max. because current new HDD pricing is 15€/TB at the low end.

What you also need is SMART output. Watch out for high uncorrectable errors, writes and whatever. I'd never buy a drive without having seen its SMART data.

Backup your data regularly and the risk should be very small.

I’m probably too paranoid to use someone else’s old storage media, but if I did I suppose I’d erase any existing partitioning, create a new partition (or multiple if you wanted them for some reason) and go to town.

Curious to hear if anyone has better best practices than that.

One anecdotal data point to consider... They might be really loud.

I am running a few 3TB drives that were formerly enterprise SAN drives, cleaned and sold for cheap. They work just fine (we won't talk about game loading times here), but they are very audible clacking away when the head seeks.

Hitachi Ultrastar 7K4000 HUS724030ALE641 3TB 64MB cache Internal Hard Drive

Use badblocks to test

dont: Used HDD are not worth the risk/cost as the chance of failure outside of warranty is too high. Maybe: Enterprise SSD's, sometimes you can find low ware SSDs that are cast off from enterprise organizations at reasonable prices, often more so if its used SAS as average consumers need an HBA to utilize the drive. Be cautious as some SSD's are discarded because of firmware bugs that cause early failures.