Confirmation on enterprise structure for shared Procurement model

As per our decided upon approach for Procurement model for our project we have created few stand alone requisition business units (we have assigned Requisition Business function to these Business Units)
We have another set of business units, which should be Payables business units for these requisitioning business units (These have a business function of invoicing enabled)

Now as per our observation:

1. For completing the Service provider relationship (i.e selecting a payable BU for each Requisition BU), unless we enable the invoicing business function for the Requisition BU, the Payable component does not show up under the service provider setup at all.