Follow the below procedures:
1. You should connect SIM300 module to selected microcontroller using serial communication(Rx & Tx Pins & GND)
2. To measure the speed of the vehicle you need some kind of interface to microcontroller (you can get the speed of the vehicle from vehicle clusters)
3.No need to interface any external memory, as the controller will have enough flash memory for your application.Just save a predefined message & call the function when ever vehicle speed limit is crossed & send it through GSM network to number of mobile numbers srored in the code.