Please help me to answer this question:
Do we need an S4 and potentially even an MDG for sending purchase info records to SAP Retail when coming from SAP Ariba (Contract Management) or is it an easier and better or simply possible option to send purchase info records directly from SAP Ariba into SAP Retail?
In a short term, why do we need S4?
SAP retail is not a product by itself, it is either an SAP ERP with Retail industry add-on installed, or a SAP S/4HANA (which has retail industry with no need of an add-on).
So no, you do not need SAP S/4HANA to interface Ariba to SAP S/4HANA or SAP ERP, nor SAP MDG. You need to gather Ariba tech consultants and SAP ERP tech consultants in a meeting so they discuss how the info records can be exported from Ariba, and define the better way of interfacing to SAP ERP ( iDocs , use of SAP PI/PO if already in the landscape...)