Marriage to the Prophet ﷺ
The Arabic proverb states:
ليس الخبر كالعيان
Narrative can never be equated to first-hand experience.
After hearing about all the virtues and merits of Muhammad ﷺ from the rest of the people of Makkah, Khadijah RA was given the opportunity by Allah Ta’ala to witness it first-hand.

Hence, it comes as no surprise that she was left greatly impressed, to the extent that she now went against her initial intention of staying single and turning down the noblemen of Quraysh; and instead opted to marry this embodiment of nobility ﷺ.
Furthermore, she did not wait for him to propose. Doing away with meaningless customs, and with the motive of securing the best of spouses; she initiated the discussion on marriage, albeit through her acquaintance Nafisah.
Ibn Sa’d reports from Nafisah:
I said, “O Muhammad, what holds you back from marriage?”
He replied, “I do not possess that by means of which I may marry.”
I asked, “If that is taken care of for you, and you are called towards wealth, beauty, nobility, and compatibility, will you not accept?”
Thereupon he asked, “And who is that?”
I replied, “Khadijah.”
He asked, “How is that possible for me,” to which I responded, “That is in my care.”
He responded, “In that case, I will do it.” [Subul al Huda wa al Rashad (quoted from Al Sayyidah Khadijah)]

The Nikah
Rasulullah ﷺ then spoke to his uncle, Abu Talib regarding marriage and narrated to him what had transpired. They agreed that he should marry her.

An interesting point to note here is that the Prophet ﷺ was 25 years old, he was old enough and wise enough to make his own decisions, yet in the absence of his parents, still saw it appropriate to consult with his uncle regarding the Nikah with Khadijah RA.

Consequently, they approached her uncle, ‘Amr ibn Asad (as her father passed away) and put forward the proposal. The uncle gladly accepted it and married her off to him with the dowry being five hundred silver coins [Ibn Hisham]. At the time of their marriage, Rasulullah ﷺ was twenty-five years old and Khadijah RA had reached the age of forty.

Her Love and Reverence for Rasulullah ﷺ
Al Fakihi reports in Kitab Makkah from Anas RA:
Nabi ﷺ was with Abu Talib when he sought permission to leave and go to Khadijah. Abu Talib granted him permission and sent a slave girl (Nab’ah) behind him.

He instructed her, “See what Khadijah says to him.”

Nab’ah recounts, “I saw something amazing; Khadijah barely heard him approaching and she already came to the door, held his hand and hugged him… ” [Fath ul Bari (quoted from Sayyidah Khadijah)]

Children from the Perfect Marriage
Six children were born out of this wedlock. The first amongst them was al Qasim (hence the agnomen of Rasulullah ﷺ, Abul Qasim), followed by his four daughters in the following order; Zainab, Ruqayyah, Umm Kulthum, and Fatimah. Another son, who was named ‘Abdullah, was born after Zainab RA.

Zaid ibn Harithah RA
Zaid was among the slaves of Sayyidah Khadijah RA. When she observed the love that Rasulullah ﷺ had for him, she handed him over as a gift to him. Thus, she was the cause of Zaid becoming the adopted son of Rasulullah ﷺ as well as his early acceptance of Islam.[Al Isabah]