Well, if you do it how I said, you might end with something similar to "Zombies ate my Neighbors"-style, you only need up down left right, for everything.. but thats just me, I have seen alot of zombies topdown games, and I really don't like them for being just so generic :/, but yeah spriting all is alot, but how I see game design is that if you are going to make something make it the best or atleast try it, hell you are spending it alot of time making a game just to see people hating it :/.