Multichannel quantum defect theory simulations of excitation spectra to autoionizing high-n Rydberg states (n = 13-150) are presented for Ar (spin-orbit autoionization), H-2 (vibrational autoionization) and N-2 (rotational autoionization), including the l-mixing due to homogeneous electric fields (Stark effect). The calculations, the first of their kind relevant to the ZEKE (zero-kinetic energy) photoelectron spectroscopy excitation range, are compared with previously published experimental results. Although in some cases the lifetimes derived from calculated Linewidths are sufficiently long for the states to be observed by delayed pulsed-field ionization, they are generally found to be too short in the highest-n regions (n > 80) to account for the very long lifetimes observed experimentally (tau > 10 mu s), pointing to the importance of alternative stabilization mechanisms. The effects of rotational channel couplings in H-2 and N-2 are investigated; these are very weak if both channels are above the Inglis-Teller limit, but show significant effects if only one channel is strongly l mixed. In H-2 it is found that a window resonance is preserved in the presence of a strong field. In Ar, ortho-H-2 and N-2 fine-structure of the hydogenic manifolds is predicted, and the distribution of intensity and linewidth amongst the fine-structure components is investigated. The non-zero quantum defects cause a lifting of degeneracy in the manifolds between different m(l) components. It is proposed that this would cause a reduction in m(l)-mixing by inhomogeneous fields as the homogeneous field increases.