Here's my Mark III Iron Hands Librarian:
I've used a Librarian shoulder pad, and a halo-style affair as the hood. Hopefully the pose does the rest!
...and while not fully Mark III; Medardus here might offer some ideas.
If you're after a kitbash, I'd suggest you use a Grey Knight torso back (to give you the psychic hood; and look for a Librarian shoulder pad (horned skull). Other details that'd work well for Librarians are a loincloth, purity seals, scrolls and keys.