Thanks for the examples hrst, indeed they are quite good for 1600 considering it is a colour film.
It is a working fabrics factory, so there should be "some" light. Half of the factory is modernized and half of it is old soviet style. So in some areas there should be plenty of light while others could be quite nitty & gritty.
I have two 35mm bodies. Fill flash is an option. Lenses are Nikon- 50mm f1.8, 105mm f2.5 & 35mm f2.