Hi Ratha,
There is no limitation in viewing the POs in ECC even when SRM 3.0 does not exist because you are using ECS and SRM POs are copied in ECC 6.0
We have just done the similar project successfully. Please consider following points which we had considered looking at business requirements though your business needs may be different:
1) Create a role for SRM super users in ECC for them to view the POs in ME23N.
2) You may consider customising SRM to do GRN for OLD SRM POs using SRM7
3) Open SCs can be recreated in SRM 7
4) Workflows need to be rebuild and takes maximum efforts
Regards,
Ravi