I am seriously considering a '11 S5 which Vossen wheels and a black grill. Even the Horch is blackened ... Will buying the car with the custom wheels and grill reduce its value later?

 

Modded cars; even if only slightly, generally reduce a cars resale value. That being said, wheels are subjective. What looks good to you may look like cottage-cheese-thighs, to someone else. Same with teh grill. They're both cosmetic, so, easily reversible (you can find Audi-rep wheels for almost pocket change, on the interwebz).

GO FOR IT!!!

Worst case, you'll hafta cough up a couple/few hundred to get that last $2k-$3k outta someone else, later on down the line.