it's not really based on or around anything. the government (the secret service, more specifically) gives precise specifications to the manufacturer, and the manufacturer must build something around those specs. they then make it look like a cadillac... kinda like a racecar is designed in the windtunnel, and then gets audi/bentley/peugeot heqadlights... without lights you'd never recognise them.
as far as those specs go, they keep a lot of stuff secret, but armored body (including roof and floor), runflat tires, a pressurised cabin (overpressure inside keeps gas outside, some tanks use similar systems in case of chemical attack), all sorts of air filtering devices, state of the art communications... no idea what this costs, but a maybach (unarmored) goes for what, $400k? so this has to be close to 7 figures. i wonder how much they bothered to protect bush... maybe they just gave him a bus ticket