I've used the Starter kit as the basis of my first Orks and still use parts in other orks. What I found:
1) The WierdNobz is easily accepted as a count-as wierd boyz. Some
40k player don't even know it's not a
40k model
2) The Iron Boyz are slightly more muscluar, but still blend well with Ork Boyz parts. Some of the Iron Boyz have partial arms attached to the torso, but you can do some clippiing to make it work.
3) The Brutes are actually a bit bigger than Nobz, but can still count. I've used them for Warbosses, Nobz bikers, and just plain normal Nobz
Pics incoming:
1) Infantry. I don't normally differnitate between the ironjawz models, but the heavy armor makes them stand out a little I think
2)
WIP shot that shows the upper arm pieces for those boyz. The first is straight IronJawz while the one-armed one is wielding a Ork Boyz arm
3) Brute as a warboss. I really need to go back and greenstuff him some nipples one day: