There is one piece of hardware where a bad choice is invisible until the exact moment it matters, and this is it.
Capacity: twice, not ten times
Buy roughly double what you need now. Too small and you start excluding folders from the backup, which is how a backup quietly becomes partial. Too large and you have paid for years of capacity on a device with a finite lifespan that will be replaced by something cheaper and larger before you fill it.
Two cheap drives beat one good one
This is the recommendation people resist most. Drive failure is not really a quality question — every drive fails, the good ones just fail less often, and “less often” is no comfort when it is yours. Two copies on two ordinary drives survives an event that one excellent drive does not. Spend on the second drive before you spend on a better first one.
The warranty tells you something
A three-year warranty against a one-year warranty is the manufacturer publishing its own estimate of the failure curve. It is not a guarantee of anything for your particular unit, but across a shelf of similar-looking products it is the most honest signal available.
When not to buy
If you already back up to a drive and somewhere off-site, and you have restored a file from each this year, you are done. Buying a third drive is not an improvement — running the restore test again in three months is.