I do love a good zombie story but there's a few things that can ruin it. Darkvision, for one, because then you have to resort to magical darkness which feels like a railroad; blindsight, which beats even magical darkness and you have to pull shit out that defeats that, which feels like an Amtrack;
these things make it hard because, in my experience anyway, the scaryness comes from not knowing what's beyond the sphere of light, in the dark, creepy corners of the city streets. If you plan on a survival horror game, you need to play up the things you'd normally ignore, like smell and touch, and especially sound. Fortunately, it's dark, and you can thusly ignore a great deal of sight until your players specifically interact with whatever it is that they're looking at. You should probably ban races with darkvision, and maybe even low-light vision, which seems like a bit of fiat, but it will enhance the experience (provided your players agree to it), or make those races worth a level adjustment, because it becomes infinitely more relevant. Pick up Libris Mortis, Heroes of Horror, and Dungeonscape, as those three are your new best friend. Dungeonscape is to add a bit of normalcy to the horror. You can throw in a zombie society, that is actually afraid because their citizens had this sudden desire to go kill a bunch of people and harvest their brains, which is really good for horror after a long bout of zombie stomping.