how rare are power Brakes ?

They're not rare. Less common than manual, but definitely not rare.

My '72 Challenger has power disks, as does my '71 Satellite Sebring Plus. So did the '74 Dart that I parted out.

Both of my '71 Dart GT's had manual drums (10", as per the GT package), and my Duster has manual disks.

So, of the six musclecar era mopars I'd had, half of them had factory power brakes. One A, one E and one B. :D The Challenger also had power steering and A/C, but other than that it's just a 318 car, the only other major options were the bucket seats and floor shift console. The '71 SSP has a 383 and is pretty heavily optioned, power steering, power windows, rear defrost, strato-vent, buckets and a buddy seat, but no A/C (Washington state car). The '74 Dart had a 318 and was a power steering and A/C car. It had the high back bench seat and the deluxe interior panels, but that was about it.