A good place to start would be using plague zombies and gangers (Arbiter gang rules?) and use more realistic models for the military. Then you could just adjust the rules to how you want the game to play. I'm always abusing rulebooks to get new scenarios and find it works pretty well. Not to mention the game plays exactly like you want it to.
Also, I'm currently working on a zombie skirmish game system which I'll eventually get around to posting. It's not meant for competitive play though. It'll have a more RPG feel with a GM (ZM=zombie master?
) but not nearly as complicated rules.