Here are two hard drives that I recently had to recover for a client. The client had been convinced to buy a used computer with used parts. Purchased a year ago for approximately $200.00 and about $100.00 in services she showed up at my office with one dead and one failing drive. The only drive that was eventually recoverable had 487556 hours on it, approximately 8 years of constant use. The other non recoverable drive had an even older manufacture date. I asked if she had know about the age of the hard drives, she indicated that she had no idea.
There is a lot of discussion on the life span and MBTF (Mean Time Between Failure) of hard drives, but if given a choice, I would choose a hard drive with less than almost 8 years of usage on it.
In the end the client paid for a new computer, and my services just less than one year since the purchase of the used computer. Used computers do not make financial sense, in many cases, such as this one, you do not know what you are buying.