Which MacBook Air should I buy?

Hello
 i’m planning to buy a macbook air,  i am searching since a week to know which one should i buy? And i was a little bit confused to understand what is the difference between macbooks that are released in the same year but have different model number such as:
MVFN2LL/A
MVFM2LL/A
MVFJ2D/A
And i want to know one more thing about macbook , as a teacher i need to use microsoft office so shall i order it to be installed on my laptop for extra fees or i can simply buy it online or what? 

[Re-Titled by Moderator]