To answer the "Boost new vs old character" thing, when you boost an old character, all of your items and bags get removed, and you're given all 22s bags. You're also given 280% flying(if you don't have it), a unique fully blue set of class armor(only obtainable otherwise from salvage missions from a garrison building later), and any professions you have are boosted to the cap of last expansion(600).
A new character gets all of the same treatment, minus the profession boost. In other words, you save about 6k gold from not having to buy all of riding training that you will by leveling a new character(this is also a great way to start on a new server if you were so inclined). The profession thing isn't that big of a deal since in WoD all profession items can be made at lv1 of the profession(so you're not gated from making things). You will take much longer to make them, but you can still do it.
Regardless of which you boost, you lose all of the class's abilities and talents, and slowly get them back again as you quest at the beginning of the expansion. By the time you get your garrison, you'll have everything back(you can just get back everything instantly if you want by swapping specs/queing for an instance, though it only takes roughly an hour the regular way).
