PLCs are overkill.
I've used ADC cards with PCs before, but I think there are serial and USB devices now. I haven't looked because I generally use a microcontroller.
The real issue with using a computer is that someone needs to write a User Interface to tell the peripheral to turn on and off or to read the temperature.
All you need to control a power device (pump, heater element, etc) is a "peripheral" with a 5V output and then a Solid State Relay. They can be controlled with 5V.
Getting temp into the computer requires an Analog to Digital Converter channel, one for each sensor. And then just use a thermistor which will give you temperature as a 0 to 5V signal.
I have purchased the auduino microcontroller board for $32 for use in my own system. It is designed to be used by non technical people like musicians and artists. I have not yet used it myself.
I will be sharing the code I write for the GUI and the microcontroller, when I write it.