Well the Horizons and Horizons beta won't disappear. So my thoughts are that if the lifetime pass is still on offer when you are ready then go for it. If not then it wasn't meant to be and you can get the DLC. If you haven't got much progress or don't mind a reset then give your account to a friend at Horizons release date for £25/£30 and then buy the Horizons yourself since that comes with the main game. It's a bit annoying to start from scratch the day of a new expansion but its a good way of saving money if you arn't already worth millions and ranked King with the Empire