Yikes! I would agree that you should get a second opinion. It seems to me that you could get that done for around $2000.
As mentioned, you could look for a wrecked one and swap engines.
It's ironic, when I mentioned recently about me looking at a 420, you messaged me that your car was going strong. I hope that didn't jinx you.
There are alot of good cars out there for under $15K. My theory is, the less you spend,the less you loose. The trick seems to be, trading out before something big fails. There are a ton of good S Class Mercedes out there betwwen $5-10K.
Your situation is why I'm being so cautious. After reading your post, I think I might know why the 380SE I looked at idled rough.
We just need to be careful.Good luck.
91 300E 120K
90 300SE 275K (sold)
92 BMW 525iM 120K
90 BMW 525iA 175K
85 300D 175K (sold)
84 300SD 245K (sold)