So if the drive passes smart test and crystal disk info as well as two passes of read/write, what is the chance that it's safe for me to use as a "write once and stash away until need to get a file" kind of thing even if it has like 2yrs of worth of non stop use in a server in prior life?
I think the chances are pretty decent. I have several old hard drives with 60000 hours on them that still work, even some with bad sectors that have not grown. And I'm not going to even mention the old SCSI (not SAS, i mean UW-SCSI) drives I still have around that seem to spin up fine. If the data you want to archive on them is very important however, I would take some additional precautions to make sure you don't lose that data should something go wrong. You could consider:
1) making some parity files so that if bad sectors show up in the middle of a file, you can reconstruct it.
2) make multiple copies on different drives, or some other cheap backup location offsite or cloud?
Basically, all the standard precautions one should take when considering important data, regardless if you're putting them on HDDs with 0 hours, 6000 hours, or 60000 hours.