somehow funny that a electrician asks if he should have a push button and a LED but already knows a RPI can be a PLC??
well from the classic PLC there was only digital I/O,
insofar there a RPI would fit.
but usually a PLC never has a multiuser multitasking GUI operating system.
but the times changed, now PLCs have analog I/O like the process control systems, there are softPLC running even on windows....
a PLC and its visualisation/operating station are 2 separate boxes linked.
how you play that on RPI?
use it for the operation / trending / development only
and use a ARDUINO as PLC
it can run 24/7 and do its job, and the RPI would be used only, if operation is needed ( setpoint change or even program change )
you can write a multijob / timeslice environment for arduino,
with a ( low priority ) serial menu for operation.
use RTC and timed jobs ...
i not know what program you play on RPI as PLC, but i think
can start with python and learn GUI programming ( for operation.. )
and after some good time play you can have your own under 100$ ProcessControlSystem http://kll.engineering-news.org/kllfusi ... tvalve.jpg