It is usual to convert a number of analog inputs in sequence. The analog to digital conversion is normally faster than 1mS, so you can convert all your 8 inputs in less than 8mS - I would think in your application this is more than fast enough.

Unless you're going to be making 100's of these devices, I would think it would be much cheaper to use an 'off the shelf' PLC for your application. At least you would not have to worry about a/d converters, pcb's, 8255's etc! Just purchase the required modules, wire in the sensors and some relays, program the ladder logic for your application - job done and most likely CE compliant to boot. In the time it takes to choose the required ic's and design and test the pcb, I would have the application written, the hardware built,tested and ready to send to the customer using a PLC.

Not to dissuade you from using microprocessors, but it is hard to compete with something that had millions spent on the design!