You can still find some drives in working shape that are believe it or not are 15+yrs. old! At the same time you can see a brand new drive give out in much sooner then thought.
As far as drive temps you wouldn't need anything well below 30C while it wouldn't be advised to allow drive temps to continue over the low 40Cs either. Keeping an eye on the temps to make sure those stay within reason is a good idea as well as monitoring those for other hardwares(cpu, video cards).
Backing up the backups of the backups is a formula to keep in mind since even a storage drive can fail. Simply relying too heavily on any drive isn't a good idea whether internal or external since the drives generally used in the external enclosures are typically the larger capacity in the budget series models.
I think everybody is in agreement on the idea of regular backups and having a disaster recovery plan inplace just in the event of any hardware failure. Even if your drive never fails all the time you have that particular system running assume it will!
That's the best way to be prepared in case it does. There's never any 100% sure fire way to prevent any drive from eventual wear or loss of data.