AGV (Automatic Guided Vehicle)

That depends very largely on what you want it to do. Should it: (a)automatically react to its surroundings in order to perform a predefined task (b) follow a pre-set course (c)just go where you tell it to in real-time?

If (a) you want a powerful computer like the RaspberryPi to process inputs such as images, and possibly also a microcontroller such as an ATmega (which is used by the Arduino) for more accurate control of the motor/s than is possible with a multitasking OS such that which you might need to run on the Raspberry Pi.

If (b), you you might be able to cheat a little and just draw a line on the ground for a basic line follower. These can be made with a Microontroller, or just a simple circuit with one Op-Amp IC (can provide a link to the circuit for this upon request). If lines are a no-go, see (a).

In the case of (c), just the standard R/C toolkit should work. Transmitter, servo, speed controller. The speed controller might need to be fairly beefy. You could use the control circuit from a small servo connected to some MOSFETs to make a DIY high power speed controller (will rotate continuously with the fedback removed).

That entirely depends on what you want to do, and what computer you use to do it. I remember there's a special Open Source OS for automonous vehicles. Can't remember the name, but a bit of searching should find it.

If you're just following a line, or a buried loop, you doen't really need a computer, just wire the sensors through amplifiers to the motors, so that if the vehicle is off track the motors will correct its path.