How to represent Pre-Heresy World Eaters within the Warhammer 40,000 game system?

For anyone thinking of creating a force of Pre-Heresy World Eaters, and wanting to go beyond a modelling project and assemble them on the tabletop, this has to be one of the most important questions. It's certainly an issue which appears frequently within the forum community, and was one that I spent a great deal of time deliberating over! So, I thought I would share some of my experiences here, and offer the best advice I can in order to choose an appropriate codex or set of rules.

First of all, perhaps the most important issue concerns what exactly constitutes 'Pre-Heresy', as this is a carte blanche term which can cover a great period of time that saw many changes within the Legion. Are you setting your force as the one which took part in the Great Crusade, and helped create the Imperium as we know it, or the blood-maddened psychopaths of Angron who took part in the assault of the Emporer's Palace? Do you want to represent the 'War Hounds' as a newly created regiment, or perhaps something in between? The character of the Legion varies widely over the 200 years + which constitutes this period, and so perhaps the most apt method is to categorise the World Eaters into the various stages and mark its transformation through the ages.

The information for the summary below is taken from the IA: World Eaters, the Horus Heresy book series (various) and the Horus Heresy Collected Visions books.

Early-Mid Great Crusade
Which Codex? Codex: Space Marines
Prior to the discovery of their Primarch Angron (which did not happen until much later in the Great Crusade) the World Eaters (or War Hounds as they were then known) were a Legion much like any other. Although they had a bias towards assault, they were still comprised of Astartes who filled other roles within the legion, such as tactical and support troops. From the evidence we have of the Legion at this time, Codex: Space Marines will allow you to field the legion from this era with some accuracy. It is possible to represent the assault-orientation of the Legion through use of veteran assault units (Vanguard) and motorised units, and even special characters using the 'counts as' rules. I would also recommend this rule set for unification-era and very early Crusade War Hound forces.

Late Great Crusade
Which Codex? Codex: Blood Angels
This period is characterised by the discovery of Angron and the changes he began to bring to the Legion once he was instated as Primarch. As such, although the Legion still had a role for generic marine units, there was more and more emphasis put into close assault units. The Blood Angels codex is ideal for this for several reasons: It allows you to represent the notorious World Eaters Berserkers, marines who had neural implants added (counts as BA Death Company), while still using standard marine units. It also allows a large amount of assault troops (which are a 'troops' choice in Codex: BA, unlike in Codex: Space Marines), and some hideously powerful characters (such as Dante, Lemartes and Corbulo) which will boost the assault power of your units still further. Obviously, these characters should not represented as their Blood Angel namesakes, but simply use the 'counts as' rules for special characters as I have here for World Eater Captain Sevruk (who is 'counts as' Lemartes). If you are a player who enjoys creating narrative and fleshing out your force, this will also give you a great chance to flex your imagination!
This is my personal favourite period, and one which I would recommend, as it allows you to not only field a distinct pre-heresy force with the character of the World Eaters, but also one which is differentiated from the post-heresy Khorne legion.
Alternatives? Codex: Chaos Space Marines or Codex: Space Marines
The World Eater Legion was not in one place during the Great Crusade and was spread throughout the galaxy as part of various expeditions. You might decide that a Company or Chapter of Marines of this era were separated from the Primarch at this time, or perhaps had a Terran commander who was not enamoured with the use of neural implants, and so the Astartes there still resemble the Legion of old albeit with more contemporary equipment. Remember that Angron had to 'iron out the flaws' in the Legion at Istvaan in the same way as the other traitor Primarchs, which indicates a certain level of resistance to the direction the Legion is taking. Similarly, there might also be components of the Crusade that take on Angron's new initiatives wholeheartedly, committing all of their marines to the psycho-surgery; in this case, Codex: Chaos Space Marines would be more appropriate.
Another Alternative! (Many thanks to Raptor1313 for pointing this out) Codex: Black Templars
There are several character traits of the Black Templars which would suit a post-Angron World Eater force perfectly. First of all, the Righteous Zeal special rule represents the WE wanting to get to grips with the foe shooting them and similarly being fearless might be characteristic of the legion. However, its the Black Templar 'vow' system which can give a great semblance of the World Eaters in combat - specifically, Accept any challenge, which forces units to charge if able to, and then gives them the 'preferred enemy' special rule (now very powerful in 5th edition). Even Suffer not the unclean to live might be useful in that it could represent relatively slower but more savage attacks.
Some of the special characters could also be converted into WE special characters with only a bit of imagination. Finally, I think the only other point to consider is that not everything fits - the BT neophyte/initiate system for example, but as always its up to the prudence of the collector to use their own initiative as to what is viable!

During the Heresy
Which Codex? Codex: Chaos Space Marines
We are told by the IA: World Eaters that eventually nearly every marine within the Legion had the neural implants fitted, turning them into raging lunatics on the field of battles. As such, a themed Khorne Berserker army would seem ideal to represent this. This has the added bonus of characters such as Kharn (who was surely as formidable in his pre-heresy guise) being usable.
Alternatives? Codex: Blood Angels, Codex: Black Templars
One of the main difficulties with the Chaos Marine Codex is its lack of wargear and vehicles when compared to the loyalist Marine Codices. A themed army list from Codex: BA will allow you these choice while maintaining the assault-orientated theme of the army. Looking at the rumours for the new Codex for Flesh Tearer Chapter Master Seth, apparently he will give the entire army the Furious Charge and Rage special rules! If you like spending time moving your miniatures, he might well be a good option!

What other Options are there?
Creating an army which matches the background narrative
There are alot of 'do's and 'dont's' when creating a pre-heresy army, such as knowing which vehicles which were not in use, and which extra ones are. The list is far too comprehensive to print here, I would recommend looking at this section of the Great Crusade forum to obtain more information. Much depends on how accurate you want to be - certainly, its possible to attain frightening levels of banality if you are prepared to spend many man hours studying the source materials!

House Rules
For those lucky enough to have a close-knit gaming group which tolerates the use of house rules then really the only limit is time, money and imagination! There are troop types and vehicles represented in various pre-heresy sources which are not present in any codex, and so would require the creation of special rules to run them. Perhaps you want a squad of jetbikes, a support squad of marines all armed with flamers and with jump packs, or even special character rules for someone like Dreagher or even Angron?! Making house rules for these characters is an ideal way to create a more characterful and accurate pre-heresy force.

For some examples of using house rules, please check out the following:

Bell of Lost Souls: Age of Heresy V2.5 -
Excellent mini-dex, is slightly out of date now but offers special character and specific pre-heresy weapon rules. Also includes an apocalypse formation as well as themed campaign battles to try out.

Tempus Fugitives Campaign Pack -
Extremely detailed house rules for the Tempus Fugitives own campaign weekend! Have rules for both Crusade and Heresy era World Eaters, as well as a plethora of rules for special units such as new vehicles and units! The professional looking PDF document is well worth a download, although you will need to register with the site first.

A Final Note
Although the suggestions I have posted thus far might seem quite set in their ways, one of the beauties of Pre-Heresy modelling and collecting is that it is far less rigid in its outlook than its 40k counterpart! Despite the recent influx of material on the events of the Crusade and Heresy, much is still unknown and we have been left a rather large void for our imaginations to fill. So, if you like the idea of frothing berserkers during the Crusade, or a force of stalwart tactical marines assaulting the walls of the Emporer's Palace then go ahead and make it! The above is only my own supposition and interpretation of events, and after all the Heresy happened over 10,000 years ago so who is to say what exactly is the truth? :)

The most important thing is having a force you are happy to spend time making, and are then proud of when it can finally take its place on the tabletop!