Just for a second opinion, I believe you would be fine with 128GB of hard drive space....which means you should probably get the 256GB option just to be safe. If you can afford the 512GB SSD drive upgrade as Kappy suggests....go for it! But for surfing and writing papers that would be expensive overkill for sure, unless you want to store your entire itunes library with a hundred movies on the internal SSD drive.
I purchased a Macbook Air for school last september and I got the 256GB SSD option. I also use my laptop for music production as well and I have a lot of sound libraries loaded on there for this. Here is my current hard drive usage:
60GB approx in OSX Mountain Lion files and installed programs
50GB Windows 7 virtual disk under Parallels for MS Office and Adobe CS6
80GB of music instrument sample libraries
20GB approx music projects
10-20GB of "free space"
So you can see that I am tying up over 100GB of space with music production related stuff which you wouldn't have, and I still have some empty space. My entire folder for my masters degree program is only about 10GB, and I am three terms in and all my classwork has been PDFs & digital textbooks.
Something I do that helps is I have a MicroSDXC card with another 64GB in the SD slot (in a flush adapter so I can leave it in the MBA all the time) and I have my dropbox folder on that. So all my music, movies, school material, documents, etc get stored on that SDXC card rather than the internal SSD - and are always backed up to the cloud.
Hope that helps!