Hello,
I have an area of rust on the wheel well opening of a Mercedes 240D - I know how to fix the rust but am not sure how far I need to repaint.  Do I need to go to the nearest body trim line or can I just repaint the color and clear to an area a few inches larger than the repair and then buff and polish the new paint to blend it with the old?  BTW, the old paint is in excellent, mirror like condition but I've been told it's difficult to match silver - which this car is.

I would  take a metallic paint to a body trim line gradually blending the paint. See https://www.paintscratch.com/touch-up-paint-directions/blend.htm for a complete discussion on blending.