Hi Prabhu,
PO - for many is PI with JAVA only.
However, as per SAP -
SAP NetWeaver Process Orchestration combines the power of SAP NetWeaver Business Process Management, SAP NetWeaver Process Integration and SAP NetWeaver Business Rules Management into one integrated offering.
Start reading from SAP NetWeaver Process Orchestration - Netweaver Technology - SCN Wiki
Coming to your question
1, Can we use PO as a Database.
PO is a JAVA based SAP solution(application) which requires a database for storing and managing data.
Supported database can be seen on service.sap.com/PAM
2, Volume of data is more ( as i said it is transactional data)
Depends on how you use it any application.
3, what is the minimum hardware requirement.
You do not install with minimum requirement, you go with optimum considering future.
Refer -
Installation guide for netweaver stack
If you are going on using HANA then
2039820 - Sizing for SAP Process Integration - Advanced Adapter Engine (PI-AEX) , Business Process Management (BPM), and Process Orchestration on SAP In-Memory Database (HANA)
Check quick sizer.
Regards,