the second world isn't that scary. It's just another world(and much nicer than the Moon, and a lot further from the ending). Just go through it already, and remember to get into a lot of battles on the first island you end up on (they're easy fighting and drop tents like crazy. You wont' need to buy any until you need to use cottages)

If you need magic, make someone a mage. It's best to level your mages up first thing so that you can have awesome physical fighters with lvl5 black magic(or lvl5 call or lvl5 white^^)

On the note of AP gaining, there's a castle somewhere in the second world that has stone soldiers-- you can throw softs at them and also get a ton of ap. (I think it's 2 ap per soldier, and they come in large groups, plus they pay off enough gil to keep buying softs (which are located in a convenient shop inside the castle itself. yippee!)

in the third world you could probably choose either, but probably won't need to (seriously, I mastered like a ton of job classes before I left that castle in the second world. especially the black and white magic ones)

You shouldn't really *have* to level up (the part I remember levelling for was when I was going to the ancient library place so I could learn lvl5doom) levelling AP is much more important, imho.

You probably will be able to buy any spells you missed in the second world (and much much nicer onesDDDD) and well, I think there's a time for the summons later. The first ones aren't too necessary by the end of the game(I think you can only get the bought ones and ifrit shiva and ramuh in the first world)