Need help on choosing Sensor board

I am a software engineer and new to Sensors.I am doing some experiment with MQTT protocol.I basically want to choose some sensor board which will be cost effective for low budget experiments.I am trying out a Motion detector sensor which is attached to Sensor board as in with Arduino uno.I need some help to buy the right Hardware for implementing this.For Arduino Uni ,Can I buy the Arduino Uno from "Arduino Uno - R3 - Original Made In Italy – India Buy Online"? Also for Ethernet shield ,can I buy from ""?Should I have to buy Ethernet shield for sure when I buy Arduino UNO? Is there any other Sensor boards other than Arduino UNO which is simple to configure,support MQTT,cost effective suitable for low budget projects?

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