You should go unlock Ninja as soon as you're able - that first involves leveling PGL to 15.

-This is what I do. Especially since you cannot trade used gear to other players. If you know you're going to level other classes, keep it for those classes until it is spiritbonded, however.
-You should use whatever gear is best. ilvl isn't important until you hit 50/60, and by then higher ilvl usually always means 'better'.
-It gives experience, as for that you don't need to.
-Raid gear, especially Final Coil of Bahamut, gives out ilvl 130 gear, which is good until 55-ish. You can probably find other people doing the older raids, however, if you look hard enough. Ask around. However, if you just do the story quests, the game will automatically give you appropriate gear from the quest rewards... I think they give up to ilvl 110...? I think. The 51 gear is around ilvl 115.
There is still plenty of content to do without the expansion if you don't want to buy it right away, and many people are still doing it. (Not AS many of course, but the crowd is still there).