Restrict camera rotation

First, define the range with these 4 variables:

 float minTiltAngle = -45f;
 float maxTiltAngle = 90f;
 float minTurnAngle = -60f;
 float maxTurnAngle = 60f;

Then, Immediately after setting _tilt and _turn, you could clamp them like so:

 _tilt += Mathf.Clamp(_turn, minTiltAngle, maxTiltAngle);
 _turn = Mathf.Clamp(_turn, minTurnAngle, maxTurnAngle);

For your "v to reset" problem, try putting this in the Update function:

 if (Input.GetKeyDown(KeyCode.V)) {
     tilt = 0f;
     turn = 0f;
 }

Update gets called every frame. when the player presses V for the first time, GetKeyDown for the key V will return true. If the player keeps holding it, this function will not be true for subsequent frames. If you want to check every frame whether a key is being held down, use Input.GetKey

All put together: using UnityEngine; using System.Collections;

  public class MouseView : MonoBehaviour 
  {
      public float Sensitivity = 0.125f;
  
      Vector3 _prevMousePosition = Vector3.zero;
      float   _tilt = 0;
      float   _turn = 0;
      float minTiltAngle = -45f;
      float maxTiltAngle = 90f;
      float minTurnAngle = -60f;
      float maxTurnAngle = 60f;
      
      void Update () 
      {       
          Vector3 mousePosition = Input.mousePosition;
          if( Input.GetMouseButton( 0 ) )
          {
              _tilt += -( mousePosition.y-_prevMousePosition.y ) * Sensitivity;
              _turn += ( mousePosition.x-_prevMousePosition.x ) * Sensitivity;
              _tilt += Mathf.Clamp(_turn, minTiltAngle, maxTiltAngle);
              _turn = Mathf.Clamp(_turn, minTurnAngle, maxTurnAngle);             }
              _prevMousePosition = mousePosition;
              transform.localRotation = Quaternion.Euler( _tilt, _turn, 0 );
      }
      if (Input.GetKeyDown(KeyCode.V)) {
          tilt = 0f;
          turn = 0f;
      }
  }