What year? I'll assume it's a 123 (77-85).
The smoke you are seeing may be the result of oil from the oil return tube leaking on the exhaust. This is pretty common. There are three little bushings that the air cleaner housing mounts onto and they break. There is a under the housing that the tube fits into. With the broken bushings, the housing can jump off the tube allowing oil to hit the exhaust. If that's all it is,$10.00 and you are good to go. Sometimes the bracket holding the bushings breaks that's another $60.00 (DIY).
Worn motor mounts usually are the root cause of the above ($50 DIY or $150-$200 tech).
Howver, if the turbo is leaking oil, you could be looking at $1000.00 plus.
I wouldn't buy any MB without a thourough PPI. Unless I was buying a parts car, because that's what you could end up with.
Protective or not, ask the seller if they would give you $5K for you wallet and everything in it. I bet they'd say no because they don't know what's in it. You should think the same way.
Low miles mean nothing if the oil hasn't been changed frequently.
84 300SD (sold)