1) I prefer M4 - better control than AK. I stay away from the SG-552 and the Aug, as they just cost more, for inferior performance. You shouldn't need the scope.
2) With a helmet, you can take around 5 shots in the head from an MP5, whereas without, one or two and you're gone. Armor only covers torso, though, but when you get hit there, a certain portion of the damage is taken from your Armor Points, and not from your Hit Points
3) Filling up on primary ammo is usually advised, except for sniper rifles and the M249 (PARA). Secondary ammo almost never needs a fillup.