In this paper we present material specific calculations of superconductor-normal metal heterostructures using density functional theory combined with a semiphenomenological parametrization of the electron-phonon coupling. In particular, we calculate the quasiparticle spectrum of different normal metal overlayers on a Nb(100) host. We find that the Andreev reflection leads to the formation of momentum dependent quasiparticle bands in the normal metal. As a consequence, the spectrum has a strongly momentum dependent induced gap. We develop a model to calculate the superconducting critical temperature from the thickness dependence of the induced gap. In the case of Au/Nb(100) heterostructures we find very good agreement with experiments. Moreover, predictions are made for similar heterostructures of other compounds.