Pastrami and mustard are key ingredients. Yellow mustard, deli mustard, honey mustard, I'll take any mustard. Corned beef also works in place of pastrami, but only when pastrami is not readily available.
Other toppings such as lettuce, tomato, onion, sauerkraut also add to the sandwich, but are not a necessity for achieving deliciousness. Only maximum deliciousness.
I don't know. Quite a lot, since I usually have a glass of something for breakfast and lunch, and more during dinner. That's not even including the fluid contained within food...