I'd go for the new
CSM kit definitely, and probably go with a 30k upgrade kit from
FW to kit them out:
https://www.forgeworld.co.uk/en-EU/Space-Marine-Boarding-Assault-Upgrade-Set
The breacher shields are very Iron Warriors, and would definitely make your berserkers stand out, and there's a big flat space on them for Khornate icons to be painted on. Rules-wise obviously the shields would be ignored.
Forgeworld also sell a chainaxe set but you can get those in lots of places.
New
CSM kit, boarding shields, Chainaxes, maybe some MKIII helms and I think you'd have some pretty neat and unique looking berserkers.