When doing an army the how and what you do is down to choice, to a greater or lesser degree - but no, you cannot take the Emperor.

However there is also the why.

For me I like my actual model choices, their options, and how I build them, to link back to (and be informed by) some background information - before D6s I was known to roll D20s and D100s, so there will always be that roleplaying narrative element with me and what I do.

So, for the Iron Warriors there are some statements about them from the Horus Heresy Book 3 - Extermination that I used for this:

The Brethren of the Iron Warriors - The Lyssatra: The Brethren of Thunder - aka the 'burned men'

Obscure and cult-like and with an obssession for destruction and devastation that others consider impractical and insane I knew I could have fun with this, from Siege Captains who'd had accidents with phosphex, to generally selecting more esoteric weapons like volkites and disintegrators just because I could, as a recurring theme of having some of the maddest of the bad really appealed - a dumping ground of a Grand Battallion where Destroyer type behaviour is not only welcomed but is actively encouraged.

Unit Organisation and Structure Within the Legion:

High attrition means Battalions etc. just fold into each other as required - forget glorious and storied unit histories (let alone individuals) - you'll go where you are told, join who you are told to, and kill whom you are told to: this means having units with mixed marks of armour fits, they get thrown together as required.

Techincal aptitude - they can and do modify their equipment, customarily practical in approach they did so to further the functionality and cutomised their personal panoply of war: this means having mixed armour parts that they repair as they go, and those different odd resin upgrade helm and shoulder pads etc. fit (quite literally), and it isn't to just look cool (although it can do), it will have been done for a practical reason. This also fits wirth the arms in the newer plastic kits, where the special/heavy weapon, and close combat weapon sprue arms are not going to match marks perfectly.

Regards armour merely aesthetic (note, not fuctional) damage is deemed irrelevant, they only repair bewteen warzones: so bloodied, burned and battered is the way i.e. lots of weathering, streaks, and with chain weapons...let there be blood.

Stor-Bezashk - the specialists assigned out from their own Grand Battalion to others, and are even better at siege warfare than "normal" Iron Warriors, and they also like atomic weapons, phosphex shells, and gravitic munitions (who doesn't?) - a throwaway line in Angel Exterminatus is that their livery is bronze cloaks - this creates interesting modelling opportunities such as bronze cloth, or bronze mail, or bronze scalemail even, or for background on where a character may have come from, or where the Saturnine or Leviathan Dreadnought had their very large gravitic weapons fitted and maintained.

Other books: Age of Darkness: Iron Within, Iron Without; Ironfire, Angel Exterminatus

Horus Heresy short stories or novels in which the Iron Warriors feature prominently. These are filled with nuggets of potentially visual information, such as Warsmiths have cloaks as part of their panoply (chainmail for Toramino and Krendl, cloth for Dantioch), Stor-Bezashk have bronze cloaks, the men of Lochos have long scalp locks to the Delchonians having whorled facial tattoos to Itherak Mountain men having red hair (Kroegers people), to Vedric Tyrperchs having forked beards.

Or a most senior Captain to a Warsmith is called an Iron Palatine as an honorific.

That sort of think can be modelled and painted if you wish and you'll find loads if you read carefully enough.

Practical Applications

So in a practical sense how does this inform things when it comes to the models? Well, take the below, my first choices for my Primary Crusade Detachment, my High Command choice (a Warsmith) and a Command choice (a Centurion):
 

So for the Warsmith it became simple - the stock one, or the Warhammer World one, do not have cloaks - Warsmiths have cloaks so a cloak I have added. This has made me consider other positions and aspects for facing that I would not have done otherwise.

They can also have a cyber-familiar and have IN10, and with that -2 to the check from said familiar (on cognis signum checks), it is a no brainer to take one - so added a cyber-familiar that adds to the model - well, I have some to chose from and I am still deciding.

I want him brutal looking but also practical and hi-tech - so the spike is removed from the crusher and the addition of more servo arm type things is imminent. He is also supposed to have a cortex controller and a cognis signum and all the other models that have those have aerials on their backs, so my Warsmith is going to have hi-tech aerials on his back, just behind the servo armour mounting bracket so they are out of the way and are less likely to snap off.

You get the idea...

With the Centurion it was more I liked the new model as a base but...

He can take a thunder hammer, and as an Iron Warrior Command that can be a Graviton Crusher instead - so I converted one out a spare Warpsmith hammer head, an old resin graviton gun, and lengthend the powermaul haft using some spare handle from an old crozius from the MKIV Chaplain from Betrayal at Calth, and made it the same length as the thunder hammer on the resin siege breaker model.

The cloak was too flaymboyant for me, especially for a 32mm base size model, and not very practical, but a smaller one taken from Deathwatch may work. Edit: been fiddling, it very much does.

The main thing was the MKVI Iron Warrior sergeant helm: the bullet crest is amusing and very Mad Max Bullet Farm but for an Iron Warriors sergeant? But for a Captain of the Brethren of Thunder, in the Hateful 88th - no, that fits - it really fits (after you drill out a neck hole anyway).

That then led to the combi-disintegrator - he has the graviton crusher to hit vehicles (so no need for the combi-melta) and he'll be shooting out to 24" so he could go with either a boltgun or disintegrator squad - maybe he developed a love for them as a veteran and that became an obession - is that why he is in the Lyssatra? Did it Overload and take out his eye and thus that is why it is bionic, or did he remove his eye so he could see destruction across all the spectrums of light?

Does he keep the spent bolt casings from his most memorable kills and add them to his crest?

As for background beyond that - well, he is the Warsmith's second so he will be an Iron Palatine.

Anyway, you get the idea but remember that whilst some ideas may end up not working on a model it can be worth keeping the idea and bits for tryng on another one - things that didn't work out for these guys will be used on a Master of Signals, a Siege Breaker, a Praevian, a Legion Champion, a Moritat, and a Herald. I am also converting the standard Warsmith into a Centurion in Cataphractii armour.

Yes, he is going to have a cloak.