I don't have the new Chaos codex, as I got the last one as a gift brand new just before the new one came out, so I didn't bother buying a new one...
From a fluff-to-tactics perspective only, let me relate what I think, as from what I understand, their legion doesn't get any of the cool special rules the Black Edged Chaos Space Marine Codex gave them...
Fluff source for me will be strictly Storm of Iron and Dead Sun Black Sky. Two stories which feature Iron Warriors and one depicting them in a more homey environment (DSBS takes place on a daemonworld, where Iron Warriors fight each other, instead of Imperium dogs =P)
The Iron Warriors in both stories feature heavily in Dark Mechanicum titans, who wage unholy war far above the space marines below. In SoI, the titan simply barrages the Imperial palace from a distance, adding to the salvoes the guns on the ground are unleashing. Iron Warriors in these books also utilize a lot of daemon-posessed howitzers and such.
So, to that, I would say, on offensive battles, take defilers and vindicators (one of one two of the other, whichever way you prefer, give vindicators daemonic posession, because the Iron Warriors like daemon posessed guns), on defensive missions take defilers and havocs (because Iron Warriors don't defend, they just attack passively by drawing the enemy into the teeth of their guns, and autocannons, lascannons, missile launchers, heavy bolters, all work best when you put a bunch of one type into a squad...), Raptors are good, fluffwise, as they would be required to assault a wall as much as tie down an offensive unit bringing ranged hurt on the IW main force. Land Raiders can be useful, but it's up to you whether to sink a HS slot with it, or boost a terminator chosen squad (if it's HS, give it daemon possession if you have nothing to stuff it with).
I personally play Imperial Fists, and the only thing we get is "Stubborn" if we take Lysander (I grew up near Lysander, NY... go figure). Yes, yes. Growl about longstanding hate between our rival legions. I'm not trying to sabotage your legion's success, just pointing out that, you can make your army siege friendly, you just have to know where to look (or
).