Vimal,
1. Can we enable the PPDS functionality in ECC 6.0 EHP7 ?
Kind of depends upon what you mean. When most people talk about 'APO', they are talking about the APO product that is a subset of the 'SCM Server' suite of functionalities. SAP now also provides selected APO functionality as an Add-on to ECC; and this Add-on is also called APO. Within this Add-on, PP/DS is available. This Add-on can be implemented against ECC 6 EH7.
SAP Advanced Planning and Optimization (SAP APO) - SAP Library
If yes then what would be extra load on CPU/Memory/Disk of ECC system.
Refer back to the Quicksizer link already provided by expert Anil, which will give you an estimated hardware load.
Best Regards,
DB49