Story-wise it doesn't matter and in the Zodiac Age Version they let you choose 2 jobs per characters, so it's very hard to screw things up, so just go with whatever looks/sounds more fun/character-friendly to you.
That being said, some of the jobs work better for some characters given their attributes and animation speeds.
My party setup was this, and I was very happy with it:
Do mind that some of the Espers in the game unlock different things for different boards so before assigning a character with an Esper, check what it unlocks for the other party members.
As for you being underpowered - some of the hunts are indeed thrown at you earlier than you can easily tackle them without grinding beforehand. Still, with the 4x speedup feature, farming a bit is not big deal, especially in the first mines area in Bhujerba.
The only three post-game things that really require you to grind and be properly prepared are the Yiazmat fight, Omega Mk. XII and Trial Mode, otherwise everything else can be beaten if you're party is in 60~ level range at most.
If you're "maining" just one party this shouldn't be an issue, but I'd recommend going out of your way and levelling up your entire party evenly; having a spare team helps a lot with some bosses.
Vaan- Shikari / Foebreaker: Daggers can apply status ailments, and Foebreaker allows for heavier armor (not really necessary) and the most useful technicks in the game.
Balthier- Uhlan / Machinist: So much for flying enemies.
Ashe- White Mage / Time Battlemage: If I had to do it all over again, I might split this out, since Ashe had quite a bit to do. Then again, it made her far more than situational, especially in the late game in some of the more dangerous areas.
Basch- Knight / Bushi: Combination attacks galore
Fran- Archer / Red Battlemage: Decent damage and spells + Ardor. I used Fran as my player-controlled character to keep her at range.
Penelo - Monk / Black Mage: So broken it's not even funny. She's easy mode, since black magick is overpowered. Oil + Firaga == dead damn near everything.
This party made for some obvious Esper bridges. Penelo could use curative magicks, Fran got upgraded spells, etc. Espers themselves are still useless; just not as useless as vanilla FFXII.
Levels don't really matter in FFXII. Licenses are so much more important. Most of the tricky hunts and Espers have some sort of gimmick that can be mitigated with equipment and proper gambit planning. Case in point - Yiazmat fell within 30 minutes (at 4x speed) after I survived his opening salvo long enough to apply the Wither/Addle/Shear/Expose debuffs. (Of course, the game makes you earn the most powerful magicks/technicks by putting them in dangerous areas.)
The final boss is still a bastard, though. I had far more trouble with him than anything else the game threw at me.