Interesting idea, I like it
A few ideas for competitiveness:
Doubt you'll need Purity Seals on the seraphim. In 4th edition, they no longer work with Hit and Run, so probably not much need for them.
Find a way somewhere, maybe drop a PE if you have to, but get the WH Inquisitor Lord into a transport. You don't want that guy foot-slogging it when he's trying to reach CC. And try to at the very least get Carapace Armor for the Acolytes. Otherwise the unit's majority armor save is a 6+. Not good. A Man-catcher or two could be handy if you want the unit to fight leader-types. Might be worth taking a second Chirurgeon if you get the points. They still get the base henchmen weapons, and you won't need to worry about instant death from a strength 6+ wound as often.
Give one of your Inquisitors a Psychic Hood. Should fit in well for fluff, and be handy since as a Witch Hunter base list your adversaries get the option of a free psychic power that could have some nasty potential, and you have nothing on hand to stop it. Maybe the DH one since he's hanging back playing Mr. Fire-base, and has the hat already anyway for modelling.