24VDC is the accepted standard in industrial controllers, effectively what you're trying to build is what amounts to a PLC (programmable logic controller). If I was asked to build a control system for some equipment like you are suggesting, then I would use a PLC. Then I wouldn't have to worry about EMC, optos, relays etc - it's all designed by companies that have made thousands of these devices and it's all tested and certified. I'd be foolish if I thought I could build something cheaper commercially. PLCs get a little expensive if you want analog inputs and outputs.

Having said that, buying something off the shelf has little educational value, but I would suggest you do a little research on PLCs just to get an idea of what is available and at what price.