One rule of thumb regarding a nas…
make sure you can afford to get spares. New or refurbished, it’s just good practice to be ready.
Just the other day I had one of my deployed drives throw up a bad SMART log. Thankfully, for a 4-drive NAS box I had bought six drives at the start. One to use for the bulk backup of what we had initially to store, and then one that would sit idle in case one of them broke. Thankfully the RAID 5 kept things usable and it was literally just a matter of power off the nas, replace bad disk, power back on and let it rebuild. Of course, the rebuild takes a long time (eta was 1.2 days for me) but at least because it’s not a computer I’m sitting at I could let it do its thing.