I would really emphasise throughout, how Haydn's introduction differs from the classical symphony. The tempo is slow. The use of tonal ambiguity with the tonic-dominant, octave textured, double dotted quavers. D-A. The absence of the third creates tonal uncertainty which captivates the listener. It is fanfare-like and really opens the symphony.
Also, with regards to the exposition, key points to remember are the simple cadential progressions that are in place and the use of monothematicism between the first and second subjects.
This is quickly juxtaposed by the development which allows Haydn to come away from simple cadences, and explore the chromatic harmony, which tonally creates uncertainty again. You must consider the use of resolution where keys are defined by the notes that Haydn uses the resolve in order to create cadences that help the music flow.
Texturally, talk about how the instrumentation is reduced in areas, due to the desired texture/timbre, and also the restrictions in the harmonic series of the horns as they are in D, with no valves, therefore can only play the notes that are present in the harmonic series of D. This helps orchestrate and execute various different dynamic changes, which contrast the mood of the piece.
My advice is to rather than write an essay is to make simple, clear points where possible depending on what the question wants, under the headings of texture/melody/harmony/structure/dynamics/timbre etc, and indicate the significance of each within the symphony.
Good luck and do not panic, make sure to spend equal amounts of time on both Haydn and musical theatre! Don't worry because the listening is also equally weighted, and the other essay on Theatre may go just as well. I am so scared for popular music, purely for the question they may ask, as the specification is ending! Anyway, good luck!