Cool HDD

Question: Will fitting a hard drive with an HDD cooler increase its lifetime dramatically or not-so dramatically?

It's hard to tell an average life expectancy of hard drives in general. Certain manufacturers have the reputation of having build more series that were prone to failure quickly than others. Other manufacturers have a better reputation but that doesn't mean you can't get unlucky when you buy a drive from them. SCSI drives do tend to last longer because they are generally build for server usage as opposed to most SATA ones. There are SATA discs that are specifically built for use in storage though, so they might me more reliable than other SATA ones.

tl;dl: If you're that worried about disc failure, read up on which companies usually produce reliable discs, buy two or more and build a RAID 0 or 5.