As the tour leader, this is my call to make. Although many people in urban areas speak some English, language will sometimes be a problem, and thus your tone of voice and your demeanor become very important markers. Be assertive when necessary, but speak softly and sweetly. Any item that you buy on the street will probably be at an inflated price, and here you should not be afraid to bargain. And even when you are overcharged, in real terms it won't be very much.
Detergent is readily available, and the Indian brands are better formulated to work in cold water.
Given these circumstances, and the abundance of dirt in India, I would strongly advise that you bring dark-colored clothing that will hide the dirt a little better. Given our brief stay, you shouldn't need to buy much, and if you do, then you get the chance to get some ginger-flavored toothpaste. There are three possible exceptions to this general rule. The first is that if you wear contact lenses, you should bring adequate supplies of everything you will need to take care of them. One can often simply dial the number directly, and the connections are generally very good.