40x to 100x work well for viewing cells. Even the simple Pi lens has been used, see: Now I'm hooked on this world of microbes
I would not suggest using the HQ camera. For looking at cells you will need the motion control of the Openflexure microscope which is designed around the Pi camera V2. The HQ camera sensor is better, but it is also larger which means the other optics need to change. Simple ways of doing that end up rather large to fit in the Openflexure body.