Why does the Torah need to add "Kein Tarimu Gam Atem"?
Rashi: To teach the Levi'im that they are required to separate Terumah from their Ma'aser just as Yisrael separate Terumah from their crops - because Ma'aser is their (the Levi'im's) inheritance.
Moshav Zekenim (from Gitin 23b): Just like you are a Yisrael, also your Shali'ach must be a Yisrael.
Why does the Torah say "le'Aharon ha'Kohen" (who never entered Eretz Yisrael)?
Torah Temimah #1 (citing Sanhedrin, 90b): This teaches us that, one day, Aharon will come back to life and we will give him Terumah - in Eretz Yisrael! 1
Torah Temimah #2 (citing Sanhedrin, 90b): This teaches us that one should give one's Matnos Kehunah to a Kohen Talmid-Chacham (like Aharon ha'Kohen), and not to an Am ha'Aretz.
A hint at Techiyas ha'Meisim.