[kiririn]

Tape drives are open about both their condition and the condition of tapes. It’s all there in the scsi log pages, more detailed than SMART on hard drives.

Mechanically and electrically, everything is rated to last several times longer than the head

In other words, you just need to buy two used drives (one as spare) and verify they can write a full tape and their head hours and other error counters are sane. There is no reasonable need to refurbish a tape drive other than a head replacement, which is easy to do at home but so expensive (for older generations) that you might as well buy a new drive. All the testing you could hope for is done in POST and by LTT/equivalent (writing a tape and reading logs is good enough)