Hello samehaddy,
I am an electronics engineer specializing in embedded systems and power electronics. Have R&D experience before in industry and have completed similar projects here in freelancer.com as well as in elance.com. Please see my profile for sample works I've had.
I can definitely help you in designing the hardware and firmware (sketches) for the base station and transceiver nodes... For the base station, a simple Ethernet shield connected to arduino will suffice, and for the sensor nodes, will consider operation at low power and low cost for efficient use... For the relays that will be controlled by the sensor signals, i suppose they are connected to mains outlet (to turn-on/off a device)? just guessing from previous projects i encountered...
I hope to discuss with you more regarding this project. Thank you!
regards,
ian