Looking to buy a used, older MacBook Air to supplement my [rather seldom] need for a portable when my iPad won't suffice. Considering I'm not looking to spend more than $650 or so, I'm figuring I won't be able to find much other than a Rev. A or B. So, what should I keep an eye out for when looking at these? I've owned my fair share of MBP's, but never a MBA before. Thanks in advance!