SAP Siebel Integration

Hi,

Currently I am working on a assignment which involves integration of Siebel with SAP. Siebel is used for CRM and SAP for other functions such as FI-CO, SD, MM . We need to write some real time interfaces which will send customer and order info from Siebel to SAP and SAP need to send back the updates ( such as order status, customer details, pricing conditions etc) . We are on 4.7 which is having WAS. The data needs to be exchanged with XML. what we are thinking is to use "Web services" for this purpose. Can this be achieved using SAP WAS only or do we need SAP XI ? what skills will we require to write Web services ? any suggestions to achieve above ?

Regards,

Vin