Is there anything like an ARM equivalent of a QUICC processor? I know the 68360 one not only the had the onboard processor that can deal with most networking on it's own but also exposed the 68020's (I think) data and address lines to the pins so you could add more RAM if needs be. Something of that sort of idea could be ideal.
When it comes to the network topolgy and protocols you've also got to decide what sort of work you'll be doing and whether a constant MIPS or "jobs per month" is more important to you.
Horsepower does NOT appear to be a requirement here.
The main issue appears to be low-cost distributed I/O.
And the second issue appears to be integration and programming.
One downside of a "mesh" of self-contained micro-controllers is
programming them properly to interact with all the other nodes.
It almost makes me think it would be better to have a single central processor that considers all the input parameters,
and makes the decisions and sends out the data to operate the system. It doesn't take much processing power to do that.
If I were doing this, my first guess would be a second-hand PC as a central processor,
something easy to control, and with the capability of presenting a nice UI on the screen,
and some industry-standard interface protocol like I2C or maybe RS-485,
and using some standard wiring format like UTP/RJ-45 (i.e. common network cabling)
There is quite probably little I/O boards out there already available that implement this kind of thing.
This is not all that different from most industrial control systems where they have been doing this for decades.
And with systems with serious consequences if things aren't done properly.