Well, I found the problem.
I took off the valve cover, and found that the cam didn't turn when I bumped the crank around with the fan blade. The tensioner had LOTS of slack. I pulled on the slack side of the chain, and this is what I found:

close up:

So what did I do? I took an engine out of a crate (with an unknown history, taken pretty much on the say-so of the private seller), bolted it into a car, and cranked it (a lot) in hopes that it would start. I recall a few odd sounds, but nothing really dramatic. After a while, it wouldn't turn any more, as the earlier posts describe.
Now I have one engine with a bad crank, and another with a broken cam chain, and I suppose possibly bent valves.
Did I violate some cardinal rule of Benz-dom?
What about that reputation for being bullet proof? How could I have killed an engine by cranking it over too much?
Or was this engine bad to begin with?
WTF???