Orks come in 3 sizes:
Mobs of 12. These go in a truck, They always have a Nob with bosspole & PowerClaw. Sluggas are good here.
They are great when you get the charge, but suck when their truck explodes, they are pinned in the rubble. and then take a charge. If you run trucks, run a lot of them. I had 5 in a 1250 list once, and it was funfunfun...
Mobs of 20. These go in a battlewagon, They always have a Nob with bosspole & PowerClaw (pattern here!)
They are great when you get the charge, but suck when their BW explodes, they are pinned in the rubble. and then take a charge. Sluggas or Shootas here, depending if you are running 'moving pillboxes' or a straight assault force... Battlewagons are great vehicles (well, the best we got). Deffrollas rock. I like boarding planks. I like red paint. Don't put on a lot of guns (just a bigshoota or 2) think of it as a 'sled' that will descend on your enemies and dump out your army...
Mobs of 30. These go on foot, They always have a Nob with bosspole & PowerClaw.
They need cover (walk them behind Kans, grots, vehicles or plant them in cover.) Shootas are good here. Orks have bad shooting - but they are assault2 guns, and 60 dice will usually kill something! If you want them to be Sluggas 25 will work too (and they are easier to maneuver...)
If you don't want a Kan line - go with 3 Battlewagons (at least 2 with deffrollas) a Mek with Force Field, truck full of Meganobs, and some fast things (Buggies or Coptas) add some Lootas for support and it is fun & fast to play. Orks are fun because they have many different builds. They are not 'top of the list' competitive, but always fun. If you want to see a bunch of Ork lists go to: http://www.the-waaagh.com/home.htm.
I have seen Kroot run as Orks (in pictures) -- and they would be great for 'MadMax' style humans too!! Esp. with trucks and bikes!!