The main things you need are the engine, LH module, and EZL. Keep your existing transmission but re-seal it, or consider a rebuild if it has 300k on it.
If you want a 2.65 diff, find one from a 1990-92 500SL (129.066 chassis), without ASR if your 400E doesn't have ASR. Plug+play, might need to swap the sensor wire if the donor is shorter. Then get a YellowBox for the speedo correction.