This is not something I would purchase casually. The air suspension issues are well-documented. It can all be sorted out and could set you back $10,000 to do so, or maybe just $3000. Or maybe it's perfect as it is.

Then the M189 - it's a wonderful engine and highly reliable. Having said that, there are issues and if you have an issue it is prohibitively expensive to repair. I think Metric Motors wants ~$40k to rebuild the engine IF they can get the parts. The generator-water pump combination is unique to this and parts are not available. The heat exchanger on the driver side of the engine tends to get bolts rusted in place, cooling lines rusted, and contaminate the water with oil. If that's the case then fixing it involves pulling the engine unless you get really lucky and are able to get to all the bolts by removing other stuff. Then you have to have a new one fabricated. (Or bypass the whole thing and come up with a different oil cooler)

I purchased a rust-free 1965 300SE lang (like the car you are looking at except a Fintail). I bought it before I knew much about the engine, and I spent two years cleaning the fuel system and injectors and pump to the point where now the engine will start. I _do_ have oil/water contamination and I'm probably not going to address that, as my interest in working on the car is diminishing. The only reason I bought the car is because it was a LWB fintail. If it were a W108/W109 I wouldn't have touched it.

So I guess the questions are:
1. What kind of condition is it in today?
2. Do you really want an air suspension car?
3. Is it totally rust-free?

If it's in anything other than good driver condition I would look for another. Or get a 4.5 or 6.3. Or even a M130. The nice thing about the W109/W112 is the better interior, increase in wood and trim, etc. The not nice thing about the M189 is it's expensive to fix. The not nice thing about air suspension is that it's expensive to fix if it needs it.

that's my 2 cents.