If you're willing to spend the tedium re-rolling, you can get a huge jump in power right out of the gate, but be warned that both of the current banners feature poor units, so you would have to chase a good unit from the standard pool (which means about a 1% chance of a 1% chance to pull something like Noctis or Gilgamesh off-banner).
That said, you can do totally fine without rerolling, but it will take you longer to start clearing end-game content, so it's up to you. If you want to spend money, wait for the good sales, and always buy through Amazon if that's an option (buy Amazon coins, use those coins for Lapis in game, which ends up saying 10-30% on top of whatever in-game sales are going on).
If you go the re-rolling route, pull on the "Rare Summon" banner (not the current event stuff), and go for something like (ranked in order of power):
Gilgamesh or Noctis + Zidane
Lightning or Luneth
Cecil + Refia or Warrior of Light + Refia (neither of these deal damage, but it gives you the main heal + tank core)
There's also a very good banner speculated to come "soon" which may or may not be weeks or months away because our release schedule is all over the place, so waiting to re-roll is another option.
Edit: You can also check this
Ranking List for a general idea of how good your pulls are. Keep in mind some of these rankings are very debatable, but overall it's decent.
