I agree protein is protein. Just look for something that tastes good and preferably has a large amount of grams per serving. Ive used Optimum Nutrition gold standard whey for nearly 10 years. It tastes like hot chocolate.
You COULD replace meals with only powder, but its generally more healthy to eat whole foods when possible because of the other vitamins and nutrients in them besides protein. I would say the 3 most important times when your body would need protein (as far as muscle building) would be right when you wake up, after you workout, and before you go to bed.
While you sleep is when your body is actually repairing and building the tissue. Not while in the gym.