To me, your system seems to be the HMI side (Human–Machine Interface) of a SCADA system (Supervisory Control And Data Acquisition) for home use, where you want to use simple (as in no PLC in the field, just an I/O) digital or analog I/O ports, and do the processing on the same laptop that is running the HMI.
It's possible, has been done before many times and in many ways, which means there is enough room for your design, too.
Arduino is just the microcontroller soldered on a PCB, where all I/O pins of the microcontroller are exposed to the Arduino board connectors. As in the ATmega datasheet, those pins can do at most 0/5V analog or digital.
Arduino shields are daughterboards that plugs in into the Arduino connectors of an Arduino board (piggybacking style) in order to extend the functionality of those 0/5V only I/O from the microcontroller. Shields can be anything, for example relays with a transistor (so they can be controlled from a 5V digital I/O to turn on/off a 220Vac lightbulb, for example, or can be a small photodiode with an amplifier to use it for example, to automate the chicken coop door, and so on.
Yes, you can use Arduino boards and Arduino shields for fast prototyping. Some shields can be stacked on top of each other, but most can not, so it will be hard to find shields with the exact combination of I/O features you need. Most probably, after a while you will end up doing your own PCB where you put an ATmega microcontroller together with exactly the relays or the other type of I/O interfaces you need.
Also, you may want to consider using RS485 instead of RS232 for distances longer than a couple of meters. Home indoors environments are still very harsh in RF noises and glitches from all the switching power supplies and switched LED lighting, fridge and washing machines motors, etc.
Anyways, Arduino or not, sounds like you have an interesting project!
