What are the implications of "Ka'asher Ta'asu kein Ya'aseh"?
Kerisos, 9a: It implies that a Ger who converts, like Yisrael at Har Sinai, must bring a Korban whose blood is sprinkled on the Mizbe'ach, 1 to preclude a Minchah.
Specifically an Olah, but not a Shelamim. See Torah Temimah, citing Kerisos Ibid. and note 28, who elaborates.
Why does the Torah add "Isheh Re'ach Nicho'ach la'Hashem"?
See Torah Temimah, note 28.
What is the meaning of "O asher be'Soch'chem"?
Ibn Ezra: It incorporates a Ger who was already living with Yisrael (but who had not yet brought his Korban - Oznayim la'Torah).
Why does the Torah add the word "le'Doroseichem"?
Kerisos, 9a: Refer to 15:15:2:1. To teach us that, nowadays, when there are no Korbanos, a Nochri can convert with Milah and Tevilah alone.
Why does the Torah insert the phrase "Isheh Re'ach Nicho'ach la'Hashem"?