Hi!
I agree with Peter - 8051 is too slow for application like yours. What do you think about using single card pc computers? It isn't cheap, but you can build network you want (f.e 100MB Ethernet). You can also use high level programming language and speed of 32 (or 64) bit microprocessor.
And the second thing is removing modules during working. It is very difficult to do. Yes, you can remove measuring modules but what about math modules? You want to make simple calculation and then send the results to the next controller. And what's happend when you remove it?

Good luck!
Jacek