Hi Prateek,
As per the information you provided about the client, SAP ECC is the best package which will suite you clients business requirement and scale of operations. SAP ECC is the complete solution which caters to each and every need of a business.
To convince your client for SAP ECC i would recommend to create a presentation highlighting the solutions provided in SAP ECC. i am pretty much sure your client will understand the system and approve for commissioning.
Regards,