Another question about Skyroads-like moving

A few months ago I asked about how to make a character move the way you did in Skyroads. (Old DOS game) The way it works is this: You have a min speed and a max speed, say 0 and 100. If you press UP (or the equivalent) it increases the speed and you go faster. Press DOWN and your speed decreases. You can never move "back" or "forward". Essentially, you are just controlling how fast your character moves. You have no real direct control over "moving" forward/backward.

Anyway, someone answered me on here and I got a nice simple script. I'm still new to scripting (I use JavaScript) and recently started work back up on my Skyroads-clone game. The method that the previous answer gave me uses transform.translate and that does not detect collisions, and is a very simple way to move. Since then I've gone over to using a CharacterController, and am having trouble getting my ship to behave like it's supposed to. Everything I try is always sloppy and doesn't come out right. So anyway, TL;DR, I am wondering if someone could shed some light here and tell me how exactly I would go about doing this. My old script (the simple one) is below, and after that I have the current CharacterController script I'm using.

I don't necessarily want you to write me up a script that I can copy+paste with no understanding. If you want to do that, go ahead, but I will seriously try to understand whatever I need to do.

Thanks so much to anyone who responds. :)

ORIGINAL SCRIPT

 var maxSpeed : float = 1.0;
 var minSpeed : float = 0;
 var acceleration : float = .02;
 var speed = 25;
 var jumpSpeed : float = 5;
 var gravity : float = 16;
 private var moveDirection : Vector3 = Vector3.zero;
 
 var curSpeed : float = 0; 
 
 function Update () 
 {
 
 // Move it
 transform.Translate(transform.forward * curSpeed);
 
 // Accelerate
 curSpeed += Input.GetAxis("Vertical") * acceleration;
 
 // Stop it from going too fast
 curSpeed = Mathf.Clamp(curSpeed, minSpeed, maxSpeed);
 
 // Jumping/Checking if grounded
  var controller : CharacterController = GetComponent(CharacterController);
     if (controller.isGrounded) {
         if (Input.GetButton ("Jump")) {
             moveDirection.y = jumpSpeed;
             
             }
    }
 
     // Apply gravity
     moveDirection.y -= gravity * Time.deltaTime;
     
     // Move the controller
     controller.Move(moveDirection * Time.deltaTime);
  
 }

CHARACTER CONTROLLER SCRIPT

 var speed : float = 25.0;
 var jumpSpeed : float = 10.0;
 var gravity : float = 26.0;
 
 private var moveDirection : Vector3 = Vector3.zero;
 
 function Update() {
     var controller : CharacterController = GetComponent(CharacterController);
     if (controller.isGrounded) {
         // We are grounded, so recalculate
         // move direction directly from axes
         moveDirection = Vector3(Input.GetAxis("Horizontal"), 0,
                                 Input.GetAxis("Vertical"));
         moveDirection = transform.TransformDirection(moveDirection);
         moveDirection *= speed;
         
         if (Input.GetButton ("Jump")) {
             moveDirection.y = jumpSpeed;
         }
     }
     
     // Destroy and restart level if below Y position -30. (Falling)
     if (transform.position.y < -30)
     {
      Destroy(gameObject);
      Application.LoadLevel(0);
     }
 
     // Apply gravity
     moveDirection.y -= gravity * Time.deltaTime;
     
     // Move the controller
     controller.Move(moveDirection * Time.deltaTime);
 }