Hi Indrajit
Please note that you can also have this working with classic scenario as well .. where the
1.PR is going to be in ECC >> Creates a SC in SRM >> PO in ECC ..
2.For activating the PDP scenario you can refer to the doc
http://scn.sap.com/docs/DOC-2721
3. Yes by default the PR once released from ECC will have all data flown into SRM so if there are changes done in the PR and if released again then again it will send details to SRM side...
4.Attachment should transfer from PR to SC on its own if not consider implementing some notes to fix the issue as below nut honestly that should be a second criteria after you setup your PDP scenario
OSS NOTE 2022800 and 1608510
Hope that helps ...
Regards
Vinita