First PLC in the project - what's the simplest/cheapest?

Dear Fellow Engineers,

I've been working on a number of prototype systems which have suceeded (oh woes...) so now it's time to industrialize their design. In the past, we used an http/ethernet-controlled relay board from Devantech (pretty neat, simple, cheap, yet capable of switching 230Vac, etc). Additionally, it's time to add some robustness, like watchdog capability, etc. Today, my newbie question is: what's the simplest/cheapest small PLC setup to provide this functionality:
- switching 2-3 230Vac (or lower ~24Vdc) circuits on and off
- sensing 1-2 low voltage circuits (high-low is fine)
- either ethernet connectivitity OR rs232 connectivity, but preferably both. My goals are to use either of the interfaces 1) to receive msgs which will lead to toggling the relays and 2) to read a PC watchdog input and cut power if the PC fails to write over a certain interval

My idea is to get a small Siemens Simatic S7 or similar with the right extensions but I've never used any of these and I'm bamboozled by the number of different modules, their capacities and unable to read through tonnes of manuals to check if they actually do what I want to achieve. Thus, I'm calling to your experience. Once the system is selected, I will manage to get through my basic PLC programming with a little bit of luck and coffee :)

Thanks in advance!

If the PC is such an important part of the process that you need a watchdog from the PC to the PLC, why don’t you do all the automation on a PC?