I haven't decided if I want to do this project yet so I'd appreciate if someone could answer a few questions for me, hopefully that will make the choice easier for me.

1. Is it possible to have custom music play at specific places and encounters, forinstance when fighting a boss or in an interior?

2. Can I make a creature or npc invulnerable or preferably teleport away so that it doesn't die unless the player has a specific item in his/her possession?

3. Whenever you approached an Obliviongate in Oblivion the sky would change color and I think there was custom sound effects to, would it be possible to do something similar in Skyrim?

3B. If so can the effect be attached to any type of static object, item, creature or npc?

4. Would you recommend that I create it now or would it be better to wait until the CS is more stable?

Edit: Spelling.

1. You should be able to. The games music system is pretty advanced compared to previous games. There are music triggers for things like 'reveals' (A giant awesome looking cavern), along with 'Dread' and lots of other music types. You can easily rig up these triggers to play music at certain times. You can also set the music type for a cell, so it plays at the player explores it.

2. Yes. Many bosses in the game already teleport around as you fight them, scripting them to do so under your conditions should certainly be possible.

3. You can use scripting to force the weather to what you want. You could make a custom weather type with red skies and doom-clouds if you want, then force that to be the active weather, having the same effect.
3b. The effect (Forcing the active weather via script) can be attached to any item that a script can go on, how you activate that effect is up to you. (Via switch, via pressure plate, by entering an area, etc)

4. CK wont get any more stable. It's plenty stable now, only a few things are finicky for some. Definitely start now. Just start small and ask lots of questions. You'll learn so much and can make almost anything you can think of.