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.
Why does the Torah say "to Aharon ha'Kohen as if he will live forever (He did not even enter 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 - a hint at Techi'as ha'Meisim.
Torah Temimah #2 (citing Sanhedrin, 90b): This teaches us that one should give one's Matnos Kehunah to a Kohen Talmid-Chacham, and not to an Am ha'Aretz.