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I've run inquisition alone in several 1850 point games, and it is a challenge. It is pretty obvious inquisition is meant to be ally tier, but since I like running monocodex armies, I thought I would give it a try. We have some strengths, like if you run a double inquisitorial detachment you can fill 1850 points, and that's maxing all your slots. We can have a strong psychic phase, having access to everything but malefic daemonology and biomancy. We also have a really decent catalog of dedicated transports, having all the land raider variants as well as chimeras (go heavy bolters and take psi ammo). Acolyte squads are really flexible and very cheap for what you get. I recommend maxing out your squads of basic acolytes, giving them all bolters. You end up with a guardsmen squad all with Ld 8 and bolters for only 5pts a pop! Guard can only dream.
All in all it's a challenging army to play alone, just don't expect to win very many games.