Why did they mention that Hashem chose Avram?
Malbim: Hashem is known to His creations also via miraculous conduct of Hashgachah. It is not based on the Ma'arachah, rather, on the deeds of great Kedoshim. Wondrous Hashgachah rests on them to override nature. This began with Avraham; he will be the Merkavah for wondrous conduct.
Why did they mention taking Avram from Ur Kasdim?
Malbim: This is when Hashem began to nullify nature for him - He saved him from the furnace.
What is the significance of calling him Avraham?
Nedarim 32b: At first, Hashem gave to him dominion over 243 of his limbs (the Gematriya of Avram) - all except for the eyes, ears and male organ. He added Hei to his name (its Gematriya is five), and gave dominion over also these five.
Malbim: This shows that he will be Av Hamon Goyim, and also his children will be Segulah, proper for the Shechinah to dwell on them.
