Which charachter are most important to gear?

if you're already at G9 with the team, you're fine. I beat it with 3 of 5 being G8. Only toons above that were Ezra (G10) and Chopper (G9). You might have to play around with your mods to get their speeds high (+60 - 70 range), but you should be fine. If you struggle, I'd recommend trying with chopper in your starting 5 before you throw too much gear at them. Get Chop or Kanan's taunt up so that Thrawn doesn't fracture Ezra and then throw Hera's special on them so they revive should they die. Strategy I used below (admittedly copied from others):

Dispel taunt (using Ezra or Chopper)
Dispel Death Trooper (using Ezra, Chopper, or Zeb)
Dispel the other Death Trooper (using Ezra, Chopper, or Zeb)
Kill DT, DT, Commander (healer), Commander (healer), taunter, Thrawn