Setting up Centralized Procurement in mixed mode

Summary

Setting up Centralized Procurement in mixed mode

Content

We are following a mixture of the Decentralized model and Centralized Sourcing so that the buyers can gain maximum leverage on pricing without burdening the Shared Services with huge load of PO's from other BU's
Structure is as below:

MS is a Shared Services BU and would be performing the central Sourcing
MD is a Requisitioning BU and would be raising its own PO's
MI is a Requisitioning BU and would be raising its own PO's
Invoicing is BU specific (no centralization there).

Would appreciate some guidance in how to set this up like how to setup the Service provider relationships, Agreements, Account derivations, Supplier setups and anything else