Andrey,
I think there might be missing points in sales & service side of this process.
My solutions depend on change for daily using hours of this process in SAP system..
ZFL1 should be Inquiry, Quotation.. or Contract maybe.. (maybe ZFL1 and ZFL2)
ZFL2 Sales Order (maybe ZFL3)
...... Delivery, Billing
* Logic is target quantity -> order quantity (separate quantities simply)
In copy control and some necessary customizing, I'm sure your process working in SAP VMS.
According to using time of this scenario which is depend on size of firm in market, it's possible to run this process in one Z screen (ABAP). Program includes item quantity from sales doc created by dealer and few other attributes with combination base of scenario..
I wonder about something, are you using material master data or vehicle master data for this process?