ive done what you are planning to do for remote Pics to transmit their address, a 4 bit data bus and 3 analogue values to a master pic that's interfaced to a PLC
Key ideas, aim for pics with inbuilt hardware USART like the 16f877, a half duplex operation as dallas semiconductors have some great line drivers, of which you should be looking towards a serial communication in the form of an RS485 using hserin and hserout commands.
just pay attention to timing values! i had great problems to start with whereby there was more than one pic replying to different requests, or started transmitting as a previous pic was still talking.
the only limit to the amount of pics on a network is the impedance and load the tranceiver chip applies. using the max487 chip you can get up to 128 nodes, but with other chips you can get more, not saying you cannot put a repeater unit to boost the signal to get the other 128 ......

Chris.