I was enrolled in the school of engineering in the first year due to my country's quota policy in university admission, although my career goal is always to to be a physicist.
This sentence is excellent. I know you are just including it to explain the situation to us, but I think it is a great sentence to include in the essay. It lets the reader know you and your situation. It is so impressive that you did a mech.engineering program and went to the top of the class. I do not think it will reflect negatively on you that you did only three years of physics, because they will see all the physics classes on your transcript.
In answer to your question, I think motivation is the most important quality of the essay. You show motivation by specifying areas of specialization and research interest that appeal to you and also by telling details about your goals for the next two years.