Hi, I think this is a good essay. The ideas are fine already. You show that you have a good understanding of Northwestern, and I think it's personal enough for the reader to see your interest in the school.
Now are some areas I think you can improve:
+ Grammar: I have never been good at grammar, so I'll just correct some errors that stand out. Someone may jump in and do the rest, sorry mate.
"I excited at the prospect of..." => should be "I am excited about"
"Last but definitely not the least" => remove "the"
"in future. " => "in THE future"
"would be eager to contribute to the diverse study body" => you mean "student body"???
"whether in making choices bearing in mind opportunity costs or in the purchase of goods and services involving demand and supply" => I just sense something wrong grammatically with this sentence, but I can't correct it
+ organization: I do have an impression that this kind of resembles a traditional SAT-style essay. I know you are a straightforward person and it's shown in your writing - I'm one too. But you can try to make the essay more interesting and lively by adopting more varied expressions and word choices. For instance, instead of saying this:
"Student groups such as AIESEC, Alternative Student Breaks, Associated Student Government, Northwestern University Singaporeans and Friends, Operation Smile, Outing Club and Peace Project interest me"
try this:
"I dream of living in such a diverse student body, and just imagining that I will one day participate in Northwestern University Singaporeans and Friends, Operation Smile, Outing Club (blah blah) at Northwestern already makes me feel part of the Northwestern community."
=> makes your sentence sound more enthusiastic and passionate.
Of course some other parts of the essay can also be improved with this approach. Sorry for the generic comment but I think you got the idea. An essay reflects your thoughts and your personality, so I won't try to make you change your essay the way I think it should be, and I'll just leave it to your creativity.
My two cent :D
Truong Pham