Referencing the flash, there's no big deal unless you want TTL for the Nikon. The Hasselblad doesn't have an accessory shoe so it may be easier to get a bracket that will hold the flash and mount to the bottom of the camera "L bracket". The 'Blad uses a standard PC cord
Flash could be as simple as a Vivitar 283 or 285. The 285 has the ability to reduce the power of the flash. THe 283 has an optional module that does the same.