BMW 320i not reliable

In 2017 I bought a BMW 320i sport 2012 ,petrol model from a dealership in Benoni.Right now it's got few problem that I have noticed I'm not the only 1 experiencing such and at BMW outlet they dont have clue how to solve the problems rather than replacing the part that are not affordable that much. The Drivetrain issue,they dont know what's the cause Misfiring of the car that will require someone to replace fuel injectors that are R20 796 all 4 of them and also coil pack that is R4 092 . 2017 the whole engine was replaced but I'm experiencing all these problems.