Sounds like a good idea for a game.
Suggestion: Scarcity of resources should be the idea and the bottleneck here. If the number of people and perhaps enemies are going to be tracked, then the resources are going to need to be tracked as well.
Building a settlement shouldn't end the game because settlements have ongoing needs that have to be met.
Food, labor, and supplies(gasoline, ammunition, medical, and so on) aren't going to just appear out of the skies and shouldn't be randomly generated in cars and stores. Food will have to be grown by people who aren't zombies. Searches will need to be carried out by people who aren't zombies. Supplies will have to be collected and stored by people who aren't zombies.
Can those people make more bullets? Will they have to rely on primitive weapons once the bullets are gone? How will the defenses stand up to the horde when the barricades are manned by people with pointy sticks. How will your science guys research a cure with no lab chemicals? What if those guys die and come back as zombies? Can anybody just decide to research until their skill hits 12 and then boom, they can make a cure? How frequently will the zombies attack? Do the zombies die permanently or can they keep coming back like in Urban Dead?
Question: Is there going to be some kind of world generation phase like in dwarf fortress?