I opted to use an Arduino as a starting point. Using an ATmega 328 AVR on a breadboard, I could experiment various wireless technologies and see if what I wanted to do was realistic.
To begin with I was considering using an Xbee wireless module for communication, however this would increase the cost too much. What I settled on was a nRF24L01, which can be found on eBay for as little as $1.50 each. Arduino has a very good library for this and were very easy to implement.
Creating a mesh network with the nRF24L01 was actually quite simple, it has no more hardware requirements, it’s all in the code. By repeating the received message, the data is passed on to the next module in range. Of course if they’re all in range then this feature won’t be used much.