Hey, just a heads up that group ( SYNTY ) is giving away free packs that you may be intersted in for your project. It says Unreal Engine but Im pretty sure you could port these over to Unity
https://www.unrealengine.com/marketplace/en-US/product/polygon-town-pack
https://www.unrealengine.com/marketplace/en-US/product/polygon-city-pack
I also found this
https://www.unrealengine.com/marketplace/en-US/product/polygon-city-zombies
Hope these help you out :)