how rare are power Brakes ?

IMO, maybe not rare but a nice option for your car. Power anything except engine was unnecessary to many folks back then. It was all about go and cost on the low end cars. Air conditioning was the wing window (which also doubled as an ash tray) and the vent box doors. Some kids these days ask what the crank handle on the door panel is for.
My BH29H came with the pictured 9"X 2-1/2" fronts along with 10"X 1-3/4" rear as standard. Even the '74 Dart I used as the disc donor are standard as well.