ARIBA Supplier Network Integration with SAP ECC 6.0

Hi,

We are trying to integrate SAP ECC 6.0 with ARIBA SUPPLIER NETWORK, so that Purchase Order could be exchange between the system. We are looking for 2 scenarios -

(1). Send Purchase Order to ARIBA Supplier Network and push it to the supplier using EDI and returning invoice to the ECC system

(2). Sending ERP Order, i.e PO released in ARIBA Supplier Network to SAP ECC System.

My Question with respect to above scenario is -

(a). Can we Integrate ECC using PI directly with ARIBA Supplier Network or we need to use ARIBA Buyers or ARIBA Procure to Pay as well for this scenario ?

(b). What is the difference between ARIBA Procure to Pay, ARIBA Procurement and ARIBA Buyers?

Any integration example or documentation would be very usefull.

Thanks