The steel won't matter to the Zombies. The blade geometry, heat treatment, and edge will matter more in the end. S7, Sr-77, SR-101, or 5160 would all make a great short sword.
I'd get something to practice with, so you can judge the lenth, handle size, and weight that will work for you. You gotta be able to swing that thing for a while and have energy to run from the hoard.
If it's too thick and heavy for you, you will tire out, and/or it will get stuck in a Zombie. It's gotta work for your arm length, strength, abilities, etc..
I love my Swamp Rat Waki, though I have yet to encounter Zombies. At least the kind your talking about.