Both cameras could work in low light, but in different ways. The Hasselblad with a waist level finder and a neck strap is pretty steady. The Mamiya can be hand held at at 1/15 sec. pretty easily. Flash with the Mamiya 7 is manual only, there is no flash automation built in. It can, however, sync at all shutter speeds.
I just recently picked up a Metz 54MZ-4 to use with my Mamiya. It works great on manual and auto.