I would save the money and the door for now. But if you realize you do need them, you should be able to retrofit them relatively easily. This way, you have a bit more buffer for the house construction.
Good, those are not bad arguments at all - however, in my opinion, a sliding door is not the optimal solution. Especially with sliding doors, and particularly with multi-part sliding doors, there is always a gap in between that lets noise or odors pass through unfiltered. I would rather invest in a good extractor hood and bedroom doors with enhanced sound insulation.
I would have preferred an open version of the stairwell anyway... Nevertheless, a closed door will not contain fish, bacon, and onion fumes. And if you are inside the house, the children are at the age of the morning compulsory school attendance. I would also retrofit them, if at all, or reconsider/refine the details of the floor plan now once again.