while tracking down the root of a failure is good preventative medicine for many automotive repairs, don't waste your time doing it on this one. Cooling fans are at the "end of the line", and so there isn't really anything upstream that could damage them (aside from the unlikelihood of overvoltages or a failing relay that endlessly chattered). Other Focus owners on this forum frequently report bad fan motors, so it's probably nothing more than wear and tear. The only thing a previous owner could have done to prematurely kill them would be man-handling them or alot of idling/AC in hot weather.
"I'm jealous of girls because they get to wear dresses." (Todd Flanders)