Anyone else with more info on the expansion packs feel free to add your 2cents.
You basically move in and out of the expansion areas continually as you play. They break down something like this (keep in mind I retired before the last one launched, so some stuff like preferred leveling areas might have changed):
Rise of the Zilart was the first expansion, and the game launched with it everywhere but in Japan. As a result, it's almost seamless with the main game and you probably won't even realize you're in Zilart areas when you find them. Your first taste of this expansion will probably be around level 25, where some of its areas are popular leveling spots. Zilart also added a series of fairly easy missions which are completely optional but required for certain high-level activities. Also, it added 2 (or was it 3?) new jobs, but there's nothing too unusual about getting them except that the unlocking quests take place in the Zilart areas.
Chains of Promathia was the second. It added no new jobs, and most of the areas never really caught on for leveling because of the difficulty of accessing them. CoP was mainly a mission/story expansion, and there's lots of that here if you're into that stuff. Again, the missions are optional but required for certain endgame activities. You can start them around level 30 and you'll be doing them right to 75. They're hard.
Really hard. And you need to bring very specialized groups of people along with you. They're probably among the most frustrating and involved experiences ever put into a game, but I guess that makes the rewards all the greater when you succeed. You'll also probably do a little leveling in some CoP areas at 50+ if you stick with it, but not a lot.
Treasures of Aht Urghan was the third expansion. It added 3 new jobs and a crapload of new areas and activities. It basically changed the way the game is played at high levels. Unless things have changed, you'll be spending most of your time here at level 60+, and almost no time before that. Again there's an optional series of missions included. They're much easier than the CoP ones, but the rewards for doing them are nothing special.
Wings of the Goddess I don't know much about, because I wasn't playing when it launched. I know it added 2 jobs, but not much beyond that.
Basically, just make sure you have all of the expansions installed and registered, and you'll be set to move into them when the time comes.
Actually if you want to try it out, you can probably buy the original on ebay and use a "friend code" to get in for like $5. First month is free. If you like it you can then buy the Vana'diel collection 2008 for the PC. Just another thought.
Unless Square has a new program I don't know about, you won't be able to play a used copy. Each package comes with a registration number which is permanently associated with a single account.