I think you need to define for us the process or type of machine. Also the signal types. 0-20 mA, 4-20mA, 0-5 vdc, 0-10VDC and so on. Also what type of information if any is to be displayed or logged. What if any control comes from the front end of the machine? By front end I mean a SCADA package such as wonder ware. You mentioned that you want a "low cost, reliable, easily operational scada system (Iam guessing) for data acquisition and control". There is no such thing as a low cost scada system, unless you are planning to interface one or more PLCs with A computer running Visual Basic. Although the cost will be less for the software needed to provide a HMI (human machine interface) the learning curve is steep as you will have to write a ton of code that is usually provided for in a scada package such as wonder ware. The system's reliability will depend greatly on the design of the hardware interface and the programmer's ability to tie all of these sub systems together in a manner that is seemless to the operator of the machine. When you say easily operational, do you mean with respect to you, the programmer, or the operator of the machine? If you desire that the system is easily operational to the operator, then the ball is completely in your hands. There are literally 100s of ways in which to construct a human machine interface. One more thing, you better know a lot about scaling analog values in order to be successful.
best regards, PLCSAVVY