Case: deciding when to ship orders

To all experts,
I would need some advices on how to handle such situation, what setup I need to make, and what configurations can manage this kind of operations.

We run a manufacturing in the site. We receive orders from only 1 customer, which is the head quarters.
Sales Orders that we receive are not stating the Shipment Date - since we have to decide what date we can ship, in a reasonable length of period.
We have setup the Manufacturing b.o.m, work centers, routing, and have been able to run the MPS and MRP - BUT currently we disregard the date. We only consider the components quantity to make the Purchase Order to suppliers.
We have also setup the Purchasing with vendor leadtime and prices.
We have also Item Card completed with inputs in Replenishment and Planning tabs.

Now, the problems are:
When head quarters send the orders, those are automatically creates the SOs in our NAV system. We will need to give a feed back on what date we consider we can ship the orders. Calculating from Purchase lead time and Production lead time.
What steps need to do? Where / which form or report we can have this information.

Our orders are a lot. The volume is approx. 100 of SO in one day, consist of 100 to 200 kinds of Item. It's not possible that we extract all outstanding order into excel and update them, and calculate them manually. We need to get it done by the system.

Does this solved by doing a forward planning (instead of backward), but how's the concept behind it.

Please help me with opinions.

Thank you very much.