It's really good what your doing, may Allaah bless you sister
I think you should add some quotations from the Qur'aan, where applicable. For example, like you can say 'What is the purpose of our lives?' or 'Why are we here on this earth?'. Then you can quote from the Qur'aan and say like 'God Almighty states in His last testament, the holy Qur'aan: "And I (Allaah) created not the jinns and humans except they should worship Me (Alone). (Qur'aan, chapter 51, verse 56)."'
I would say, emphasize about Tawheed, as this is essential when giving da'wah. Compared to other religions, Islam is more clear and straightforward about the belief in One God. Maybe quote some verses where Allaah mentions that he is one God, or just give references.
And you see how you mentioned that the Qur'aan has not been changed, since the day it was revealed etc, you can get an ayah from the Qur'aan, where Allaah says Himself that He will preserve the Qur'aan. Also, you can add that Allaah challenges people that if they doubt in the Qur'aan, then they should produce something the like of it, although at the same time, Allaah informs them that they will not be able to do so. (Qur'aan, chapter 2, verses 23 and 24). Then you can say how many people in the past tried to write something 'better' than the Qur'aan but failed and ended up embracing Islam themselves.
Maybe you can mention some of the miracles of the Qur'aan. E.g. "He has let loosed the two seas (the salt water and the sweet) meeting together. Between them is a barrier which none of them can transgress. Then which of the Blessings of your Lord will you both (jinns and men) deny? (Qur'aan, chapter 55, verses 19-21). Then state that this is a fact, which scientists have discovered, whilst it was mentioned 1400 years ago, to an illiterate man, Prophet Muhammad :saw2: (peace and blessings of Allaah be upon him). How was he supposed to know this? At a time where there was no science or technology? Maybe give like three examples of miracles of the Qur'aan and mention that there are many more miracles etc.
You know where you asked as to why many Christians have embraced Islam, show an example. Like Yusuf Estes was a Christian preacher and he embraced Islam, so maybe you can get a quote from him and other reverts that are well known, like Siraj Wahhaj and Bilal Phillips etc.
Basically sister, whenever you mention anything, back it up with evidence and examples. I think also it shouldn't be
too lengthy because then people may lose interest. Then at the end, urge them to get a translation of the Qur'aan.
I hope that helped sister