gas fire/box/flue

I am planning on putting a flue and fire box in the living room so that we can use a gas fire instead of our currently used electric fire. The flue will be a class 2 type and I know that I am obviously limited to a class 2 fire. I will need to build a false chimney in the living room and upstairs bedroom. Do I buy the fire and hearth & surround first and work to them or do I put in the box & flue and get the surround etc to fit that. Could some with the expertise please tell me the order in which I need to do this job. Thanks...Eric