If you want
TL-Rokkits, save yourself 10 points a pop for rokkit buggies, that survive small arms fire and term/horm melee better than koptas.
Lootas fare well against
MC's, but I wouldn't rely heavily on large squads. Nids cover ground so quickly they make me want to invest in depends.
Gangsta. It's gangsta, and you know what gangsta is? Gangsta is driving two battlewagons up to a demon prince and unloading sluggas until he dies

That's gangsta style!(and no drinking tea with the pinky extended)
Oldest ork advice: Need more boyz!
Enjoy, and remember something: You SHOULD get one phase of shooting in with shootas.....templates....well, I normally play mechanized orks and my opponents don't get template weapon hits on me.
MANZ should be mechanized, but if you do that, you're better off with the entire army mechanized.