Hey Mike,
if you stick with your original Z-16 Z assembly, you can add the 80 mm spindle mount. The Z-16 assembly with its flat pitch 1004 ball screw (4 mm/revolution) will not let the spindle go down with gravity when the controller is powered off.
The new Z-20 “heavy” Z assembly gives you more stiffness but has a much steeper 1610 (10 mm/revolution) ball screw. Because of the steeper pitch, a Z stepper motor with brake may become necessary to hold a spindle when the stepper motor is unpowered. Onefinity offers one for the Elite Series since the spindle drop issue came up with the new Z-20 assembly.
Also with the flatter pitch 1004 ball screw on Z-16, you don’t need such a strong Z stepper motor. The steeper the pitch of the ball screw, the stronger the stepper motor has to be.
For the Z-16, the stepper motor is 23HS22-2804S 2.8A 1.2 Nm (Onefinity Replacement part). The replacement with a dual shaft to be able to add a brake is 23HS22-2804D 2.8A 1.2 Nm. The brake is available here. But it is unlikely you will need a brake with the flat ball screw pitch of the Z-16 assembly.
On the PRO Series with Z-20 slider, the motor is a 23HS30-2804S 2.8 A 1.9 Nm (Onefinity replacement part). The variant with dual shaft for attachment of a brake is 23HS30-2804D 2.8 A 1.9 Nm.
If you stick with the buildbotics-derived Onefinity controller, a nice option is to retrofit proximity sensors as limit switches for highest accuracy regarding homing repeatablity. You however need to be able to 3D-print parts or to know someone who can do that for you.