Generically used drives still don't look that good to me, but used enterprise grade ex server drives do. Consumer grade drives could easily be abused in heavy use like a NAS, and the used buyer would never know, plus consumer grade drives aren't going to show up on the used market in quantity when they have no issues. They are going to be true "refurbs" where some operation took problem drives, and "fixed" them. Just look at the number of drives on the bay sold as parts only.
You can't trust any drive, and my last HUGE snafu wasn't a drive failure, but some software issue that killed a motherboard raid array. I have joined the make a backup club, which is a big part of getting into NAS units.
That is the main issue, how much storage do you need, and what sort of redundancy will you be using. I can only guess right now what my storage needs will be in 6 months or a year etc, and that eventual upgrade will require 2 drives with all my old data in a mirror, and the new larger NAS, followed by upgrades of the 2 old NAS units or splitting the backup between several units.
I started with 3x new 2TB WD reds internal to my media PC, put 2x 3TB WD reds in first IX2 NAS 5 years ago. Media PC had a motherboard failure a year ago and I am almost at the end of a two week long effort to sort through all the old recovered files and duplicates in my current PC with 2x 3TB WD reds, a HGST 4TB hanging out the side and one at a time the old WD red 2TB on a USB 3.0 / sata cable. Too much info without an answer, but 3x 4TB JBOD in a NAS is plenty for now, preferred is maybe 4x 6TB in a raid5 same model of NAS.
These old Lenovo NAS units are so cheap, I just bought a second 4 bay PX4 for $99, and may get a third and skip using or make use much more targeted with the two smaller IX2 2 bay units so I can make use of 2TB and 3TB drives.
Mostly its about what turns up in the resale market, and that is driven by what enterprise level servers are getting updated.