We present an end-to-end simulator for 3D spectroscopy, which can be used to specify MOAO-fed integral field spectrographs dedicated to ELTs. This simulator re-scales either local data or outputs of hydro-dynamical simulations to model distant galaxies. We present simulations of 3D observations in the H-band, for a rotating disk and a major merger at z=4, and a large range of stellar-mass. We use these simulations to explore the parameter space, focusing on the impact of the telescope diameter, total integration time, spectral resolution, and IFU pixel scale. The size of the telescope diameter has little influence on the spatial resolution of 3D observations but largely influences the achieved SNR. The choice of the IFU pixel scale is driven by the optimal "scale-coupling", i.e., the relation between the spatial resolution of 3D observations and the physical size of the features for which one needs to recover the kinematics using this IFU, and the SNR achieved with this spatial scale. To recover the dynamical state of distant emission line galaxies, one of the main goal of such future instruments, one only needs to recover their large-scale motions, which in turn requires only relatively coarse IFU pixel scales (50-75 mas) and moderate spectral resolution (R=5000).