Definitely get Immortal Throne. It's the original game with extra features, an extra act, caravan for some extra storage and transfer between toons and artifacts. Yes, you can import from the original, Immortal Throne will do that automatically once you install it and start it up first time.
Welcome back to the game Kalimon . Yes, it's rather easy to transfer characters to Iron Throne once you install it, and I heartily recommend it. I don't think wraith shell is worthless at all. Granted, it doesn't make a difference in many fights in higher difficulties, but 1 point into it does make it significantly harder to kill. Arcane blast, however, is a skill I never put a point into: you want your lich king to spend his mana on soul blight, the skill that reduces all enemy resistances by a %. The lich king gets the soul blight ability when you have 5 points in the basic lich king skill, and it grows more powerful with every point added after 5. If the lich can cast arcane blast, you'll see far fewer soul blights cast to help you out with lowering your enemies resistances.