I see that no one has responded to you yet. Did you see the car on Tuesday?
We have a 1983 280SL, which we bought last summer from the original owner. It is a Euro model (as is the one you are looking at, because they did not import the 280SL to North America). Ours is a 5 speed, so I can't comment on the automatic. Ours is also low (80,000) miles. It starts instantly and runs like a top. Very smooth and plenty of power with the manual transmission.
Our car's rear axle also hums on slight acceleration. I would be interested in what you discover about this, because I am thinking about having ours looked at, but don't want to spend big bucks for a non-problem or a solution that doesn't cure the noise. I have read that rear end noise is common to the car, because the diff is mounted to the body and is right behind the seats in the SL, but I don't know what is considered normal and what is excessive.
John R. Boyce