Anything I can think of to answer your other questions has already been mentioned, so I'll just talk about this one:

Quote Originally Posted by RogueGhost View Post
1.) Gil - ... build a bankroll, and is it really necessary?...
The only really expensive thing that you buy directly from the game or from NPCs is a house. That's a big expense, but it's a one-time expense (and one that not all players are even interested in).

All the other big expenses tend to be things that you buy from other players, and that you could earn or make directly yourself instead if you prefer. For example, there's one stage of the Relic quests that requires a lot of materia. You could either spiritbond lots of gear until you manage to make all the materia you want yourself, or if you have lots of gil you could just buy materia from other players. So, while having a lot of gil is never exactly necessary, it can allow you to trade one kind of grind for another. (One player might choose to do a bunch of gathering or crafting to earn gil so that they can buy materia rather than spiritbonding to create it, whereas another might choose to do spiritbonding to earn gil but would buy crafted gear so they don't have to level the crafting classes to make it.)

So how necessary you find gil to be has a lot to do with how much you want to avoid or speed up certain grinds by buying their items versus how much you want to be self-sufficient and make or earn all your own stuff directly. (Even crafting itself, which is often seen as a way to make gil, can range from being a big gil-sink if you're buying all your materials, to costing nothing but time if you're gathering all your own materials.)