Installing OSX on a 2009 MacBook Air

I recieved a 2009 MacBook Air from a friend and I am wondering what the best way to install OSX is. Am I able to use his AppleID to install Mavericks? Am I then able to sign out of his acct. once the installation is complete? I don't want his ID to be linked to the computer anymore. Or should I purchase an older OSX version (I don't even know how this works on an Air) with my ID and then upgrade to Mavericks? Or something else? Thanks for the help.