Physics and gravity of my 3D model

Hello, im new to Unity3D and I have 4 big problems :) watch the video with my problem

video: http://www.youtube.com/watch?v=OaS3F0KiYSM

1 As you can see, gravity doesnt work at my tank, my tank is light as a feather and more times takes off from the ground, not to mention that the tank should not climb the hill

2 I dint know why, but when i start "flying" with my tank my turret becomes unattached as you can see

3 How can I add some physics tank, for example, when I go to the hill so the tank appropriately tilted backwards ... or tank leans after passing a small hill

4 Question i think its last for today :) is somewhere some nice intro Unity3D that is displayed at the beginning of the launch such as http://www.youtube.com/watch?v=a5yQkHCfMEs CryEngine or Unreal Engine 4 http://www.youtube.com/watch?v=dLNTXEz0WYM ... CE3 and UE4 have a very nice intro :)

There is my script for controling tank:

function Update() {

 var steer=Input.GetAxis("Horizontal");
 var gas=Input.GetAxis("Vertical");

  
 var turnSpeed:float = 50; 
 var turnAngle:float;
 var moveDist:float;

 if ((gas!=0) && (prvy == 1))
 {
     
     moveDist=gas*speed*Time.deltaTime;

     transform.Translate(Vector3.forward*moveDist);
     
 } 

 if ((steer!=0)
 {

     turnAngle=steer * turnSpeed * Time.deltaTime * gas;

     transform.rotation.eulerAngles.y+=turnAngle;
 
 }
 

}

And here is my hierarchy of my tank:

Thanks for your help