-i think the cheapest sub is $11. might be 14
-only multi you need is for dungeons. after you unlock a dungeon, you choose it on a list and it puts you in a queue til you get yr team matched up
-you can start your own party, and it'll out you on a browseable(?) list. you can define exactly what types of party members you want, state what your objective is, whatever. very customizable and easy to use
-very little. if you do the side quests in towns you'll get more than enough exp, and even without them there's plenty to do to gain exp. levels come very fast in the main story
-only content i found was impossible to queue into was the super dungeon from the base game, called the coil of bahamut. it's still very hard to do so you'll prolly have to go outside of the game to put together a party that'll work thru it. if you end up liking the game it's definitely worth doing bc it's got a good story that's important to the lore. otherwise everything is still easily accessible, though you might have to wait till peak hours to do some of the other optional dungeons
also if you can get onto hyperion server do that bc that's where all the cool bore ppl are (and also it's a heavily populated server)