What are you using the network for if all the Arduino's independently log the sensor data to an SD card?
If you want to give each arduino a static IP address then either hardcode it into the sketch before upload or store it in eeprom memory so it survives power down if it's set dynamically using buttons during sketch execution.
There is no need for sending information from server, except maybe for confirming messages.
30 to 200 network enabled Arduinos are likely to swamp your network/server. Might be better for the server to periodically request data from each arduino in turn.