Hi Ankur,
The extent to which you want to use the functionality of PS depends upon the Business needs. There is no set rule or guidelines as to what should be the minimum use of SAP PS. It will also depend upon how a business runs its projects in the legacy system and whats the expectations out of SAP PS.
In my view minimum would be
1. Tracking timelines of the project using well defined structure.
2. Planning and Capturing Cost.
3. Budget Control
4. Resource Management
Hope it helps..
Cheers
Samar