Procurement BPO - we are on our way to choose a global partner - what lessons can we learn from those who outsourced indirect (i.e. non strategic) spend and other items i.e. help desk, before us?

At Qantas in Australia, we are in the process of finding a global BPO partner to manage around US$1b of non strategic spend, as well establish / manage - a Help Desk, internal / external portals, compliance, analytics, Spot Buying and Master Data Management - any ideas or lessons learnt would be appreciated.