Siemens PLC Powersupply

Iam working on designing a system with Siemens S7-300 PLC.Please let me know how to decide on the powersupply required.There are 3 types of PS(2A,5A,10A).Do we need to do a Backplane current calculation for each and every module as in AB?

You can use ANY 24V power supply. It does not have to be one of the power supplies in the hardware catalog.
You can leave slot 1 empty in the HW Configuration. It has no function, no matter if there is an S7-300 PSU installed or not.

Just pick a decent quality regulated power supply and you are set.

To figure out the currect requirement, you need to sum loads....each card and cpu have their current demand stated and look for the worst case...
Usually on an average stand alonde setup a 5amps for cards and plc + hmi is well enough and best practice would be to add another one for separate load to power...
Jesper is right for any 24vdc plc but Siemens ones even the low cost LOGO series power supply are great for immunity to noise, spike and overvoltage issues...I have seen many other brands connected on the same line failling where the Siemens remain ok where you had to replace a whole plc rack because the designer saved 25$ for a cheaper powersupply