Forge World seems to be the key for the Word Bearers during the Shadow Crusade. They have a lot of specific models and rules : no Argel Tal (yet ?), unfortunately, but Erebus, Kor Phaeron, Zardu Layak, Gal Vorbak, Mhara Gal Tainted Dreadnought, the Ashen Circle (even if those are somehow prior to these events), Lorgar and upgrade packs for the common Marines.
World Eaters models are fewer, but there is a pretty Rampagers squad, Angron, Kharn and Contemptor Dreadnought (alongside conversion bits too).
Is it mainly for modelling or playing ? With 40K or 30K rules for the latter ?
What I remember from the books are Possessed Word Bearers (the Vakrah Jal lead by Argel Tal) on the front line with (a lot of) World Eaters disorganised assault troops, supported by Land Raiders (decorated with Ultramarines bodies, not all of them dead yet) and Litanies, Rhinos and Dreadnoughts.
Diabolists (including Erebus) and cultists were on Calth. A few Daemons would therefore not be off topic, I guess.