On the subject of fluid, is it a good idea to replace (partly) water with milk? Milk here is really cheap and tastes awesome (Normandy cows...)
I wouldn't, because they perform different functions. Water provides hydration, milk provides nutrition and calories. Drink one or the other, depending on what you need at the time.
Also, remember that more is not better -- not with any of this, not with training, not with nutrition, not with hydration. You want an appropriate amount -- more than that is too much.