REPAIR DETAILS

I am a car dealer, a wholesaler, car is inside my garage. I have some experience with German cars. This one I bought misfiring on all cylinders pretty much, starts then stalls or runs for few only if you hit the gas several times, have no throttle response, has a backfire noise at front, may or may not be the engine., was flickering lights on the cluster due to low battery charge in the trunk. checked front and it's good, recharged the back and it stopped flickering. At some point I had code M4/7, P0300, P030X on most cylinders, replaced all coils, still has a misfire on 1 or 2 but the way it sounds doesn't make sense. Even with a misfire it should stay on, but it struggles to stay on, and am not sure if the noise am hearing is a knock or simply because the cylinder isn't firing, as I have had that in other v8 benz before so not sure. Also had code B210D before I charged the rear battery. Previous owner had ton of amplifiers and crossovers in the trunk professionally installed and he removed the speaker box only when he sold it. So I am leaning towards an electrical issue maybe, or an engine issue. I need some diagnosis.