A magnetometer to provide the direction the car is pointed would be more valuable on a car than an accelerometer or gyroscope. An accelerometer would tell you when the car is accelerating or if it flipped over, but a gyroscope would be pretty useless unless you're planning on doing flips or such.
You can find a few parts on eBay that include both a voltage regulator and an I2C level translator to make them fully 5V compatible. If the item does not state it is 5V logic compatible then be aware that you will damage it with a 5V Arduino if you do not add your own level translation.