The only E50 I show for the 210 was produced from 01/96 to 08/97. This E50, if it is a 2000 model would probably be either a fake, or was still produced at the seperate AMG facilty, so it would be considered a "tuner car", not a factory built car. Although this is somewhat offset by the fact that MB is a majority shareholder of AMG.
For authentication I would write or email AMG or possibly Mercedes would have record of it if it went directly from the factory to AMG. In some cases the car will actually leave the factory and go to AMG without even being completed, as AMG will modify so much of the car it makes no sense for example to install interior components or driveline which will just have to be removed later.
In your case you would want to be wary of a car which has just had badges applied, ground effects or body kit added, and some engine trim changed, maybe some suspension mods, this is very easy to do, you are wise in trying to research it, but I would have done this before purchasing it, especially if the owner has no supporting documentation to show it's "real".