Better than DH? In certain areas (anti-tank) possibly. DH (grey knights) are CC beasts. Sisters are NOT marines. Marines have a better T, I, S. Marines are capable in CC as well as at range. Sisters specialize in short ranged firepower. Sisters get to use Faith abilities, which make them absolutely amazing when used properly. Necrons will give you headaches. As for the guardsmen, you could induct them and bring along their Russ. Land Raiders? I think only an Inquisitor Lord can take one, and that's a LOT of points that you could potentially lose to a lucky lascannon shot. Assassins require you have an Inquisitor or Inquistor Lord, and I'd say choose either the Eversor or the Callidus.