Hi Eswar,
Please find the answer below for your questions.
1. Requirement of seperate OPC server is actually dependend on the SCADA software you are using. Sometime OPC server is installed with the SCADA software as a component. You can refer this post for better understanding. Integration with SCADA / controllers
2. It depends on the customer, on which opc they are familier. Kepware and OSI PI are used mostly, but matricon is also used.
3. MII cost is depend on the no of user your customer required. cost MII lisencing cost vary depending on the no of user. (no idea about NW)
4. PCo is freely distributed with SAP MII and no seperate lisencing is required for PCo. It comes with the same lisencing of SAP MII.
Hope it will help you,
Regards,
Suman