You'd be pretty set in any fairly modern apc, tank or assault veichle, as the huge majority are not only utterly unopenable from the outside save with some serious firepower or cutting equipment. And most also come with built in vacuum seal doors and anti chemical/radiation/poison attack air filtration systems. You could even buy your own FV-432 in operational condition for around £10,000 used, and by used I mean it sat on an mod base for five years doing the odd training run. Seats 14, has all the hazmat gear, dual-fuel power-pack engine. Definately my veichle of choice in a zombie apocalypse.