yes, acually the sales for Halo 3 in 24 hours stills hold the records over all games including GTA 4 and the second is halo 2 so both of these hold quite high standards.
wait for the next halo
Yes
Yes
Follow the objectives
If you are looking for a detailed guide, I would try YouTube for a level walkthrough.
no
Beat the game on legendary
yes anyone could
You complete the game
Depends on what level You beat it on.
I would say Halo 3 ODST
you will get elite flight armour