He has a 1993 300CE.
All 1993 300E's / 300CE's / 300TE's are misbadged.
They all have 3.2 liter M104 engines, not M103's.
I think you are suffering from the two major problems this engine has from the model years 1993-1995:
1. You are likely leaking oil at the right rear corner (passenger side) of the head gasket, and the smoke you are seeing is a result of the oil dripping on the exaust manifold. The leak is very small, which is why you are not seeing your oil level go down.
2. The rough idling and popping are likely due to a failing engine wiring harness. Engine wiring harnesses from 1993-1995 are bad. The insulation flakes off the wires, causing your car to idle rough, misfire, hesitation on acceleration, backfiring through the intake manifold (the popping noise), etc.
Both of these problems are very, very common, and any dealer or Mercedes independent has problems done hundreds of the above repairs.
Try the search function on this site for "wiring harness", and you'll have enough reading to keep you up all night.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".