I think other forum members may be hesitant to respond because the answer appears to be obvious. In some instances, use of a used part or component may not present a problem; i.e. a set of used wheels for instance. OTH, used electricall and/or fuel related parts may present a plethora of problems. There is an old saying; "There is the PRICE and then there is the COST".

The price of a new fuel pump for a 560SEL is not cheap. OTH, jerking around with a used one can easily eat up one thing you cannot replace: YOUR TIME. Buy a new fuel pump and that is one less thing you have to worry about. Let's face it, if you are out on an extended road trip, THE LAST THING YOU WANT TO WORRY ABOUT IS A FUEL PUMP. Sitting on the side of the road at night waiting for a tow truck is not an enjoyable experience (been there, done that).

A 560SEL is not the world's cheapest car to drive (ask me how I know). The car deserves to be maintained properly, either maintain it properly or sell it and buy a less expensive car like a Ford or Chevrolet.