I'm looking to purchase a refurbished iMac from the Apple Store (21" i3 Model), and I was wondering if there are any known issues. I have a friend who purchased a 24" a few years back and he says it would run hot all the time. He mentioned that due to this he had screen burn in, a failed super drive, and audio in jack as well. I know he put his iMac to sleep when he was not using it, I dont know how often he shut it down. Are there any issues I should be aware of?