As a closet door manufacture we start working when closet organisers are installed. And in 80% of closets there are drawers in the middle. Drawers in the middle are very convinient when you have bi-folds, or very large closet, when sliding panels are wide enought not think about drawers. In all other cases we need to make extra track on the floor so you can move all panels to one side and have access to your drawer, or make more panels (4 insted 3 for example). Extra tracks and panels will increas final price for your door at least on 100$.
My advice about drawers: make them on sides and less than 1/3 of closet length (you if plan 3 panels).