  Context. Multi-wavelength studies of galaxies and galactic nuclei allow us to
build a relatively more complete picture of the interstellar medium (ISM),
especially in the dusty regions of starburst galaxies. An understanding of the
physical processes in nearby galaxies can assist in the study of more distant
sources at higher redshifts, which cannot be resolved. Aims. We aimed to use
observations presented in the first part of this series of papers to model the
physical conditions of the ISM in the nuclear region of NGC 253, in order to
obtain primary parameters such as gas densities and metallicities. From the
created model we further calculated secondary parameters such as gas masses of
the different phases, and estimated the fraction of [C II] 158 um from the
different phases, which allowed us to probe the nuclear star-formation rate.
Methods. To compare theory with our observations we used MULTIGRIS, a
probabilistic tool that determines probabilities for certain ISM parameters
from a grid of Cloudy models together with a set of spectroscopic lines.
Results. We find that the hypothetical active galactic nucleus within NGC 253
has only a minor impact compared to the starburst on the heating of the ISM as
probed by the observed lines. We characterise the ISM and obtain parameters
such as a solar metallicity, a mean density of ~230cm-3 , an ionisation
parameter of log U = -3, and an age of the nuclear cluster of ~2 Myr.
Furthermore, we estimate the masses of the ionised (3.8 x 10^6 M_sol ), neutral
atomic (9.1 x 10^6 M_sol ), and molecular (2.0 x 10^8 M_sol ) gas phases as
well as the dust mass (1.8 x 10^6 M_sol ) in the nucleus of NGC 253.
