It used to be you could buy a computer and then upgrade the hardware as you went along to meet the demand of newer operating system versions. I use a 10 year old computer that has limitations but has lasted a long time because I was able to upgrade as I went along. Unfortunately with the Macbook Air you cannot do this and if you want the computer to last to the end of your studies you will have to take advance measures buy buying more computer right now than you really need for today's operating system. Yes, it costs more, but it means that in 3 or 4 years when you want to install the latest operating system you won't get a message saying you cannot do it because you have too little "memory" and the only way you can get more is by buying a whole new computer. In my case I was able to spend another $100 when I needed more memory, but not with today's computers.
Get an external hard drive. No, get two. You always want a backup of your internal hard drive, unless you don't mind re-writing your whole term paper the night before it is due because your computer's drive crashed at the last minute and you didn't have a backup copy. You want a second drive because these nice tiny Macs cannot accommodate a large hard drive unless you're made of money, but today's multimedia gobbles up hard drive space. You will need to store your movies and things you are not using immediately at the time on an archive drive. However, again you do not want just one copy. So you want one external drive for an archive drive for overflow of music and photos and movies, and another external drive for backup. Yes, these cost money too, but it is like buying a shiny new car and then not buying insurance in case the car is stolen. A car you can always buy again, but if your photos get lost when your drive fails or your computer is stolen then there's no replacing those.