Pastrami and corned beef are essentially the same, a cured brisket of beef. Pastrami has the peppery coating and is smoked, corned beef does not have the coating and is slow simmered but can also be slow cooked in an oven. If you are corning (curing) the beef yourself you will need to allow the extra days for the curing process. If you are getting the brisket already corned (cured), simply proceed as you would for cooking a whole beef brisket. Low and slow. My preferred method is a slow simmer in water. But to do it for over 100 I think you are better off doing it in the ovens. My across the street neighbor did a non corned brisket for Hannukah a couple of years ago and cooked it in one of those cooking bags (like the kind you get for turkey). The corning does not effect how the meat cooks, only how it tastes. It turned out delicious, moist and fall apart tender! Now he was only cooking for around 15 and wasn't cooking the whole brisket of beef but two smaller ones. It only took a couple of hours to cook.
I don't know if you will be working with a whole brisket or cut up one. for 100 I am assuming you will need a minimum of 25 lbs (allowing a 1/4 lb serving per person) but I would allow extra as the brisket can sometimes be fatty and you will have some waste. The average brisket can weigh anywhere from 8 to 12 lbs so you may have anywhere from 2 to 4 briskets. So how long you slow cook would be dependent on the size of the brisket. When I do a 4lb corn beef I cook it on the stove in a big pot at a slow simmer, completely covered with water and I allow 3 hours or more. I will often add extra seasonings to the water such as bay leaf, peppercorns, mustard seeds and a healthy pour of wine to the pot (white). I guess for that many people you could have two or three 4lb briskets per professional chef stew pot and have several slow going at once. If this is your first time cooking corned beef, I would highly recommend you pick up one in the grocery store to experiment with just so you are confident the day of the luncheon.