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I would like to start a space marine army to go with my grey knights and the exorcists fit in perfectly with fluff.

Now I am trying to decide what chapter tactic would best represent space marines that have been possessed by demons and lived to tell the tale?

Ultra marines could work, really could make them for any fluff.
White scars. Suddenly they all just became dare devil bikers?
Iron hands could represent durability gained from being possessed.
Salamanders like fire, demons like fire, these guys could have developed an affinity for fire.
Imperial fists. Gained a new perspective for defending Terra?
Black Templar get angry and all zealous, being possessed could have upped their zeal.
Raven guard favor shadows and being creepers, possible side effect of possession

What do you think would best represent space marines who have been purged of a demon possession?

Depends entirely on how the army composition is in the first place. If we know the list, we can help choose the Chapter Tactics to fit it correctly.