The sales order itself should just be a record that you acknowledged a customers desire to purchase a product from you. It's best to use all of the existing documents in the system to track your orders rather than adding in a bunch of validation steps for the sales order.

There is a 'Create Tasks on SO' module, but I don't think it will do quite what you want right out of the box, so in order for this to be done properly you would need to edit your document flow. I would do it one of two ways:

1:

Sales Order  ––>  Task  ––>  Manufacturing Order  ––>  Delivery

2:

Sales Order  ––>  Task
          \            \
           v            v
          Manufacturing Order  ––>  Delivery

In the second case, the MO waits for the task to be completed as well as waiting for the raw materials to become available.

The task here is the design step, so you'll want all the information related to the design entered in the task. This might require adding fields to the task object to meet your needs. One thing you will want to do either way is add a field to the MO referencing the task to put useful information related to the design on the MO.