Is it safe to use a drive that previously had bad sectors (since reallocated via a Secure Erase)
This question could be considered 'opinion-based' however, in my opinion...

No hard drive is ever going to 'heal' over time; it can only degrade.
If it's developed bad blocks before, it will likely develop more as time goes on.

Unless you have a Time Machine constantly connected &amp; working, giving you an hourly safety-net, I'd bit the bullet &amp; swap it out now. 

Better safe than sorry.

You'll also love the speed the SSD gives you. It will feel like a new machine... or actually faster than new.