It really depends a lot on what you want the system to do, how flexible you need it to be, what kind of future expansion capabilities, etc.
From an NI product line perspective, if you are looking at using a PLC you should take a look at our cRIO solution. Like a PLC, Compact RIO has a plethora of different I/O modules. Unlike a PLC, Compact RIO is far more programmable/flexible. It can run a real time operating system with user-programmable FPGA for tight control.
If this seems like overkill for your solution, you might consider something more simple. You could probably get away with some simple digital I/O. We can provide solutions pretty much everywhere in between. You might consider contacting someone in our sales department to talk through your requirements and have them make a recommendation.