You also need to work out the speed needed.
For torque we know the radius of the brushes is about 6cm, and we want 0.3N tangential force (presume
this is for the size of brush quoted above), in which can the torque = 0.06 x 0.3 = 0.018 Nm.
[ actually probably want a healthy safety margin above this since friction is involved, say 0.05Nm ]