I think most H#D bodies go together with backs. H#X are another range of bodies that take different makes of backs. Look to begin with 31MP,40MP,50MP etc.
.PS: Forum member "TechTalk" should be here any moment with expert adcvice.
Sorry for the slow reply. The combination of time and energy have been too scarce lately for forum activity.
To the best of my knowledge — H3D, H3D II, H4D, and H5D bodies and backs are interchangeable with each other. Some features for matched generations of camera body and back may not be available when combining different generations of backs and bodies.
Hasselblad offered the option of purchasing additional dedicated bodies with factory focus calibration for a specific back. A dedicated body can store and apply focus calibration data for more than one back. Focus calibration is done at the factory using a proprietary calibration process.
The H6D user interface is substantially different from previous generations and I'm not aware of what compatibility may (or may not) exist with prior generations.
H4X, H5X, and H6X camera bodies have additional compatibility with other backs which varies with each model.
Bodies, backs, viewfinders, and lenses should be updated to their latest firmware versions to maximize compatibility. Personally, I would want to test the combination of a body and back of different generations before purchasing or have a reasonable return period for refund should any unanticipated connection issues arise.