Why did Eliezer mention the fact that Sarah bore Yitzchak only in her old age?
Seforno: To inform them that from birth he was particularly dear [to his parents, 1 thereby boosting the Chashivus of the Shiduch in their eyes].
Ha'amek Davar: In case they would think that Sarah's son was too old for Nachor's granddaughter ? he pointed out that Sarah gave birth to him in her old age, and that he was therefore not so much older than her. 2
Malbim: He was dear to his father, because he was born through a miracle.
Since Sarah was Nachor's niece, why should we think that her son is too old for Nachor's granddaughter? Rather, Besu'el knew the age of Sarah (his first cousin) relative to himself, i.e. she was born 130 years ago. He could have expected Yitzchak to be about 100 years old! However, if Besu'el's family was in touch with Avraham (or Terach) even after they went to Charan (11:31), until Avraham went to Eretz Kena'an, Besu'el would have known that Yitzchak is less than 65 years old. (PF)
Why did he repeat "Eshes Adoni Ven, lA'doni"?
Malbim: He taught that (a) Yitzchak was from Avraham's wife, and not from a concubine. (b) Yitzchak takes after his father in beauty, Chochmah and good Midos.
QUESTIONS ON RASHI
Rashi writes: "Avraham gave [Yitzchak] all of his possessions - [Eliezer] showed them the gift document." How do we know this?
Gur Aryeh: Eliezer could reasonably suspect that the family would not believe him. It is better to present one's proof at the outset, and not wait for others to dispute one's claim.