Question for Robocup

Hello!
I’m building a soccer playing robot for the Robocup competition - and I need help because I never worked with Arduino.

First I’ll buy this chip:
dx.com/p/arduino-mega-2560-atmega2560-sensor-shield-v1-0-expansion-board-dupont-line-usb-cable-set-139994?item=11
(I would like to know what is the second board?)
And battery holder:
dx.com/p/6-x-aa-batteries-holder-with-dc2-1-power-jack-for-arduino-126413?item=2

I would also like to buy those sensors:
4 x Distance (US) -
dx.com/p/hc-sr04-ultrasonic-sensor-distance-measuring-module-133696?rt=1&p=2&m=3&r=2&k=1&t=1&s=148659&u=133696
2 x Infrared Sensors like this. (The ball is a IR emitter, is this the best sensor to locate it? do I need a better one?)
dx.com/p/robox-arduino-compatible-ir-direction-sensor-module-145084?item=6
1 x Compass (Is this a good one? Any other recommendations?)
dx.com/p/robox-arduino-zduino-electric-compass-sensor-module-red-144212?item=3

1 x (Probably) a driver for 4 DC motors


4 x DC Motor

Now:

  1. Are those the best sensor types for what I need? If not, can you recommend others? (from here or DX)
  2. MOST IMPORTANT QUESTION: Is this everything I need to build this robot? (Do I need any other drivers/thing to connect the sensors and motors to the Arduino Mega??)
  3. What is exactly the difference between Servo and DC motors? What is the best one for my needs? How do I connect it to the Arduino Mega?
  4. As continue to the motor we chose on question 3 - Which omni wheels I can buy on this site that will fit in to it?

Thanks!

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