If the wall paper is in good solid condition and not contact paper or a very glossy type surface. Then a good quality sealer should be all that is required to prep the surface after cleaning it good (a green scotch bright sponge while cleaning will work to give somewhat glossy paper some bite). After the prep you could the texture it. But I would seroisly think about the removal. Sometimes tough, but many times wall paper can come down fairly easily. Maybe try some in a small area and see how it is holding on.