My view is that iteration of the core design is equally important to the the iteration that's done on the rest of it. The difference is that each 'product' in a field is a new iteration on the core design. This is what makes experienced engineers so valuable over inexperienced ones - the experienced ones know the history and know the previous iterations so that they can make the 'right' next step.
Very true, but also not something you should focus on too early.
You'll need a lot of experience for this initial design phase to be really fruitful.