I'm building a NAS (FreeNAS, Xeon E5640, ~4GB ECC) to replace my Synology 1212 (single 2GB drive) with a more redundant and fast system. Primary usage will be ensuring integrity of data that's rarely used but needs to be immediately accessible (will have offsite backups as well as FreeNAS/ZFS snapshots), with an occasional need to transcode for Plex.
The system will have SSD-based SLOG/ZIL and L2ARC, and I will upgrade those in size so they should encapsulate commonly-used files. Thus for the spinning portion of the system it is more important that it be reliable than that it be an extra 10% faster.
My plan is for either RAID-Z1 or RAID-Z2, with 2TB disks. That more than doubles my current storage needs (which have been stable for several years), so I can put the extra money into redundancy rather than capacity I won't use.
Given these goals (reliability > performance but with terrible performance being bad), what drives should I get?
Lightly used vs new
Heterogeneous vs identical models but with different batches vs identical models purchased from the same place
Aiming to keep it to $50/drive shipped or less, preferably more like $40.
Any recommendations for specific model numbers or brands or lines would be helpful as well. My habit in the past has been to buy whatever's cheapest from a major manufacturer, and that's served me just fine but trying to be a little more thoughtful this time.