Any factory production car will pass the noise limit easily, as will just about any decent muffler system. Cars with leaking or open exhaust system may have trouble with the noise limit. If you run a stock car with no significant modifications, there are no other costs up front for participating. Further, it is common for more proficient autocross drivers to invest in higher performance tires, shocks, and seat belts. Those that have become hooked on autocross have been known to spend even more modifying their cars to improve performance and handling.
All modifications are optional and none are needed to have a good time.
However, the best investment that can be made is to yourself. If you are considering modifications to your car for autocrossing, start with the driver. Then, work on the suspension. Save the engine work for last. A typical autocross course covers a large parking lot and is marked with pylons or other temporary marking materials.