Chosen dont have unique models. Just strap some extra shiny bitz on regular CSM and call them Chosen.
As for their tactics: a good way to use them it get 5 of them, give them all Melta, Flamers, or Plasma, and have them Outflank. They're one of those awesome units that can have 5 special weapons, so they work great for blasting specific targets. You can also load them up in a Rhino if you have the extra points, which almost guarantees they'll reach their target.
