Without wishing to appear rude an 11 year-old C180 Coupe isn't worth a huge amount and if you start adding performance modifications the cost (and the resulting insurance hike) may quickly add up to more than the car is worth.
If you want a more powerful car it may be best to sell your current one and put the money towards something that better fits your requirements.
If its the M111 engine the way to big power is to supercharge or turbocharge em-- its what Mercedes did. Otherwise very limited--- best thing-- a nice free flow manifold + exhaust system would make it a much more tractable car.
If its naturally aspirated then as said your options are basically looking at airflow through the engine. Exhaust and headers/manifolds are best places to start, the challenge may be finding the right components.
I have had a look around eBay but not much going that I know will fit, I have been offered a custom stainless exhaust system for 300 but as its a de cat I don't know if I should as mot time is coming.