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What type of armor would the World Eaters of used during and before the heresy? I'm guessing Mark III, but I'm not familiar with the pre heresy armors, and which particular army used what.

From Mk.II - Mk.IV, maybe Mk.V
Those are the Pre-Heresy armours. Some Legions also had some kind of variant armours of their own too, not sure which ones though.
Wouldn't be too out there for them to have one or two suits of Mk.VI either, the Raven Guard were testing them at the time.

I think they'd look all the more better decked in Mark IIIs

Well for what its worth in betrayal you've got 6 World Eater exampes:
3 in MkIV armour
2 in MkII armour (one with a Sarum pattern helmet - which is what became the standard helmet for MkV)
and a single MkIII with a custom helmet

Note MkIII is more of a close-range standoff suit, its not really suitable for actual hand-to-hand combat.

I would think in the run up to the heresy they would be mainly in Mk IV as this was the new improved mark and Horus channeled the newer tech into the legions he considered most likely to follow him.

I'll probably end up going with Mk IV then. Seeing as I would be modelling an army around the time they turned. Perhaps a little of each type. Thanks for the help everyone!