That's good that you ask these questions. I love answering them.

First Question: Turn 5 and Turn 9 refer to the first and second coil of Bahamut. A level 50 set of raids that have very difficult bosses. So if people refer to T5 or T9, this is what they are referring to.

Second Question: Your gear cannot be leveled up, your gear is stuck at that level and it is just a way to compare what items are better. For Example, Item level 60 (i60) gear is not as good as i90 gear. ^^

Third: Yes. In order to gain certain skills required to be a good Paladin, you must get your Conjurer to level 15.

Fourth: You can spiritbond your equipment and sell the materia that you make from that, you can do the story quests and naturally get lots of gil. Doing every dungeon atleast once requires you to do a quest to unlock them which will give you gil.

Fifth: Hunts, you can participate in by joining some linkshells specifically for looking for these high level monsters. These monsters are all level 50 so unless you are a Disciple of Magic, you will likely die instantly from one of the monsters normal attacks. A coordinated effort of atleast 8 skilled level 50s are required to beat them. These people do not know by heart as although each monster can only be in one area. Where they can be in that area changes at random.