Joey - The link from Niwashi is all the dungeons and raids currently in the game(with one exception for a new non-traditional dungeon from the most recent patch). It also includes a link to the quest that unlocks each of them. In this game, no instanced content is unlocked automatically - everything is gated behind some quest or other. Most of them you automatically do just by advancing the storyline, but there are many of them that are totally optional as well. In addition to the dungeons and raids, there are also a variety of trials you will be asked to complete(think of these as 4 or 8 man boss fights with important NPCs like Ifrit) As for how much ARR content, you have to finish all the pre-HW storyline quests(the ones with the fiery quest icon). Nothing else is required. The story will send you to ishgard when the time comes.

Grant - That is correct. Any exp you earn while capped is lost. If you still have content you want to do before getting the expansion you can always try leveling multiple classes to spread your pre-50 exp around. Otherwise, a lot of people get the expansion shortly before they hit 50 so they wont lose any exp. For the record, a lot of the quests right at that moment dont give a ton of exp, so you wouldnt be losing too much. When you buy the expansion you get a registration code. You add this to your current account on the mogstation website, and the rest happens automatically when you log in.