Saving documents... unless you're saving huge multimedia files, you dont need a lot of space. How much space does you computer now have? How much is used right now? Running low on space now... then maybe think about getting a computer with more than you have now... on the other hand if you use a fraction of your current space, why spend more money for more space that you will probably not use. That being said, Air and Pro both only come in 128 or 256 (Pro has a 512 model if you really wanna spend money) GB versions.
Pro has better hardware, air will get the job done if all your doing is writing, reading and viewing stuff, but lack of ram on air is something to keep in mind. Pro will handle a lot more, but its more expensive.
Not a big fan of windows (or os x really), but you can get a small light, Air comparable, machine for about same price, but with 2x memory and perhaps better processor now days.
As for discount, maybe get around $50 off,