Understanding best practices in purchasing based on MRP

Hi

Just wanted to learn from the community any best practices that one may have adopted in regards to Purchasing from MRP. A few areas I am looking for are

a). Do organisations create Requisitions or Purchase Orders directly from message processing

b).Do such orders ever go through approval

c). What happens when supplier or price is not known at MRP message processing time.

d). When different items on a single PO/Requition have different material types which require different approvers.

e). Does anyone use R3411 for message processing

Any insight would help greatly.

Thanks.

Govind