Hi Trevor,
It is not necessary to replace the EDI, Barcode & warehouse application in your scenario.
EDI & Barcode can be integrated with SAP through interface.
Warehouse application can be replaced with SAP Standard Warehouse Management which is further tightly integrated with your logistics process.
As you a supplier to major automotive organisation & they are also using SAP system for their operation, SAP implementation will bring in a lot of value & easy flexibility to meet the customer demands for you as a supplier.
Warehouse application can be also be interfaces, however it is recommended to use SAP standard for warehouse process as your logistics operations are in SAP.
But if you have an extended warehouse management processes, then SAP offer an application called SAP EWM (Extended Warehouse Management)
Regards,
Sachin Balmiki