It's tough for companies to justify supporting a car that doesn't sell enough volume to have enough of potential customers. SLKs are relatively rare compared to C63s/M3s/Porsche-anythings. Most owners aren't interested in modifying either. The lack of aftermarket support was the main reason I thought about selling my R171 SLK55 because I autocross and track the car often. I eventually found everything I needed to find to get the car to where I am very happy with it.
That being said, there are (limited) options and it really depends on what you're looking for.
Exhaust - Quicksilver as mentioned, Supersprint, or custom. I really don't see much need to pay for a brand name system when there aren't much gains to be had with exhaust work unless you remove or replace the factory cats.
Engine - Renntech has an ECU tune. Don't see the reason to risk changing anything though. The car has plenty of power.
Suspension/Handling - KW makes coilovers for the R172 as does BC Racing. The BC Racing kit also includes front camber plates. I have the BC kit on my R171 55AMG that sees plenty of autocross and track time and love it.
http://www.amazon.com/BC-Racing-Series-Coilover-Mercedes/dp/B0145NYO8G
Exterior - Piecha has a few things like bumper lips and spoilers.
That being said, there are (limited) options and it really depends on what you're looking for.
Exhaust - Quicksilver as mentioned, Supersprint, or custom. I really don't see much need to pay for a brand name system when there aren't much gains to be had with exhaust work unless you remove or replace the factory cats.
Engine - Renntech has an ECU tune. Don't see the reason to risk changing anything though. The car has plenty of power.
Suspension/Handling - KW makes coilovers for the R172 as does BC Racing. The BC Racing kit also includes front camber plates. I have the BC kit on my R171 55AMG that sees plenty of autocross and track time and love it.
http://www.amazon.com/BC-Racing-Series-Coilover-Mercedes/dp/B0145NYO8G
Exterior - Piecha has a few things like bumper lips and spoilers.