Our results show that electron-hole interactions are crucial to obtaining the correct polarizability and dielectric screening between octahedra within many-body perturbation theory (MBPT) in these low-electronic-dimensionality systems. A range of optical, photoelectron and polarization spectroscopies could be employed to further study the behavior of excitons in this material class, including Stark spectroscopy, temperature-dependent optical measurements, excitation-dependent THz and electromodulation spectroscopies. Moreover, the majority of previous theoretical studies have not included vdW dispersion interactions when modeling these systems, yet here we demonstrate their importance in obtaining ac-curate crystal and electronic structure predictions, calling for their inclusion in future computational studies of these and related low-dimensional and 'molecular' crystals such as the A 4 BX 6 family.