Why did Hashem instruct Moshe to raise specifically a Saraf?
Seforno: to remind them of their sin - the slander that they spoke with the breath of their mouths, just as the snake injects its venom with the breath of its mouth.
What is a "Neis" in this context?
Why does the Torah insert the word "Kol ha'Nashuch"?
Rashi: Because it seems, that even dogs and donkeys bit the people, 1 wounding them and causing them to become weak.
See also Ba'al ha'Turim on Pasuk 6.
Since when do snakes bite and since when do snakes cure?
Rashi (citing the Mishnah in Rash Hashanah 29a), Ramban Rashbam and Targum Yonasan: It wasn't the snakes that killed, it was the sin; and it wasn't the snakes that cured, but the fact that they looked upwards and subjugated their hearts to their Father in Heaven. 1
Ramban: And realize that Hashem kills and Hashem brings to life.