Monkey,
I have quite a bit of experience with armored vehicles. I purchase them for overseas governments. If - as a civilian and a private citizen - you want to protect yourself inside a standard vehicle, your best bet is to wear kevlar body armor inside the vehicle.
Yes, there are armored vehicles that civilians can purchase. However, the art of armoring a vehicle is a really esoteric art, employing special glazing materials (windows), metallurgy (door panels), welding, polymer products, and - in particular - design.
One of the more infamous assassinations of a military officer riding in an armored vehicle occurred when an assailant fired a round through the rubber gasket element which held the bulletproof glass in an armored frame and carriage.
http://en.wikipedia.org/wiki/James_N._Rowe
You do not want to trust your life to some sort of Mad-Max red-neckery involving welding armored plates onto your average Buick.
Not only will it be utterly ineffective against most rifle fire, but its patently unsafe - the extra weight is going to snap engine mounts, crack your frame, destroy your suspension, and cause you to lose control of the vehicle.
Armored vehicles are designed from the blank page up to accommodate the extra weight and mass - e.g the inertia that results when you attempt to maneuver them.
If you want to drop a few hundred grand on a commercial armored Jeep Cherokee or Chevy Suburban, go for it. Otherwise, buy a decent set of body armor and call it good enough.
When the zombies come, an armored vehicle - home-made or not - is NOT the way to go. You just make a slower-moving, easier-to-stop, higher-priority target.
Best,
Doc