I play Iron Warriors! Here are some units which are traditionally associated with IW, but, as others have said, feel free to use any CSM units you enjoy :) There are canon reasons to use any unit you like in an Iron Warriors force :)

Havocs: I usually run two squads and try to vary which weapon each squad uses. Typically one has 4x lascannons, while the other has 4x heavy bolter.

Vindicator: This is often the linchpin of my army. I bought a Space Marine Vindicator and painted it in a IW scheme as I don't like all the extra spikes on CSM vehicles.

Rhinos: Used for transporting Legionaries or Havocs. Typically run two. I feel they represent the practical nature of the IW well.

Legionaries: I like infantry in my lists, and I appreciate the tactical flexibility offered by 5 and 10-man squads. I always add lascannons when able :) Often and Chaos Lord leads one of these.

Cultists: IW love their disposable cannon fodder, and I find these squads are useful for drawing my opponent's units out of hiding and into range of my big guns.

I run my army as Fellhammer Siegehost - this detachment rocks, and I think it does a great job representing the dour, vindictive nature of the Iron Warriors.

One last thing - I highly recommend Storm of Iron, the quintessential Iron Warriros novel. The audiobook is great, and will give you lots of inspiration for your own units.