Given what 18K could get you in the used car market, even the used mercedes market, that price is ridiculous. You could probably buy a beater W123 and pay somebody else to retore it to an equivelant status and still not pay half of that.

"One owner" benz prices are absurd to the point that I automatically ignore them if I was looking for another one.

Low mileage thirty year old cars are another red flag for me. Either the odometer is screwed, the seller is dishonest, or it sat in storage for decades and every hose, seal etc. Is buggered and lots of things will be "frozen" and will all start failing soon after regular use.