Excluding frame, So for that weight it needs to be stable & pretty big.
So let's say another 3kg for frame & lipo & it's guts. Is this for crops or something.
3kg per motor as you stated. As for the 50% throttle @ 1.5kg you would have to sift through the specs of each motor.
& my understanding is another 50% makes the 3kg so just look for that thrust in a motor. 100amp esc's for each motor aswell.