well i have a WD 320 gb external that i bought 3 years ago, the first year of its life it lived under the bech seat of my tow truck and stored music videos and work files. i had a dash mounted laptop that ran my gps and work files so as long as i was in the truck the comp was on and so was the hard drive. i had an inverter store under the seat for ac power. now i spent at least a third of my time on back roads and dirt roads under less than optimal conditions. after i removed it from the truck it has spent the last couple of years getting daily use as my back up and utility drive. i have had no issues with it. as it goes it has its own built in fan and as long as i dont place it in an air restrictive enviorment then it never gets above room temp.
personally i dont shut off my comp except to do an immediate reboot, i dont shut down and leave off unless there is a need (i.e. repairing or replacing internal parts ect) computers and their internal components are like any other man made device, subject to accelerated failure due to cool down and reheat. once your comp comes up to its operating temp you will cause less wear and tear by leaving it running staying at temp than you will by cooling it and then heating it up again.
i have left mine on for almost 2 years without many complete shutdowns and have had no issues with it. i dont know if i would consider it the rule or if mine is an exception, but im content with it either way.
i am more than certain that there are plenty of people who would argue this, but to them i will just say go back to metallurgy 101 and reread the part that tells what happens to metals that are subject to continual heating and cooling. plastics fare no better in this environment.
if you are going to use it for important stuff i would say get a ac powered one with a built in fan, make sure it can breathe freely and you should do fine. i would also suggest that using a worm drive or it equivalent to back up anything of real importance. anything can happen at any time so having a backup that is almost impossible to corrupt is smart not matter what your saving.
good luck