The color match is difficult on 15 year old cars! Depending on the color of the car as well. MANY times a replacement fender wont fit as well ar the OE one.
I would have the OE fender repaired but $500 is very high!
IN the US a new fender is $222(OE). Have it Painted off of the car then you install it.
AFTERMARKET is just that on fenders! FIT can be terrible. I have replaced the ones on race car with the cheap ones(Tawain) only once, they just don't fit right.

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MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator