D-head doesn't have a lens motor, so it needs to be driven by the body motor when autofocusing, while some low-end models of Nikon don't have a body motor, so D-head can't autofocus on these bodies, so it can only focus manually.
The models of Nikon motors without fuselage are: d3000, d3 100, d3200, d5 100. Other models have fuselage motors, and all of them can use D-head normally.
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As for which fixed focus you say is practical, it depends on your shooting theme and budget.
Because of the conversion coefficient in the frame cutter, the choice of focal length is more important for the adaptability of the subject.
50 fixed focus is suitable for shooting outdoor portraits and some still lifes. Unless there is a lot of indoor space, indoor portraits can only be taken as busts or headshots.
35 fixed focus is equivalent to the viewing angle of the target on the frame cutter. If there are many indoor portraits, you can choose, or you can shoot some small scenes.
As for the fixed focus of portraits such as 85, because of the conversion coefficient, the shooting will be far away, and basically only outdoor portraits can be taken.