Hey Mehtabahmed,

I'm not that expert but I believe for CPU you have to check the memory you need, the type of network you're gonna use and whether you want to use standard CPU or fail safe version. For I/O modules, it is easy, it depends on the number of I/O and if you want to use safe faile modules. 

I suggest you have a look at TIA Selection Tool which you can download for free  here.

I hope it helps.

Cheers,

Dave

Hello mehtabahmed,

1.) Are all I/Os wired to a cubicle or will there be distributed I/Os?

2.) Which interfaces will be needed? Is there only one or more interfaces (busses) parallel?

3.) How much memory will be needed? Will there be many complex functions like PID-controllers or Sequence controllers?

4.) Is there a need for interrupts? How many?

5.) Are there other conditions that have to be cared for like security or redundancy?

6.) What is price of CPU?

that should narrow down the possible CPUs 

regards

CPU selection has been discussed here already. Look here:

CPU selection

Selection of CPU

Selection of CPU

how to select CPU

if you go through the answers you will also find information about literature.

regards