Hi, I have the opportuniteit to buy a 9 month old rMBP. Checked the serial number, warranty and apple care is still available. Gave me screenshot of an 'LSN' = Samsung screen. Invoice&original box etc is till there. It's an i7 (2,3GHz), 16GB, 512GB. One thing.. I'm completely new to OSx. All friends and fam. members use Windows. What more should I be aware off when buying a 2nd hand rMBP? What should I check or test, and how?