Does the microwave/convection oven come with or without the venting apparatus; i.e. can any GE unit that fits in the space be installed or does the enitre system have to be replaced? In your experience,Is it likely the existing wall bracket will match to a new unit of the same or essentially the same dimensions? And does GE still make a microwave/convection oven of essentially the same dimensions (29.5" wide, door hight 11")? If not, what do you suggest?