Welcome to SMF. You are certainly Jumping into the Deep End with Both Feet!
Small Parties are 50 or 100 people. 1500 ? That's Aaron Franklin Volume!
For Sandwiches of 1 protein, Brisket, 1.5 4 ounce portions are common. Women and Kids will usually eat 1 sandwich, Men, 2 sandwiches. If this is Plated, Meat and 2-3 Sides, 8 ounces of Cooked Beef is the norm.
For Sandwiches...1500 X 6 oz(1.5 portion sandwiches) =9000oz / 16 = 562.5lbs cooked beef / 0.5% yield = 1125lbs Raw beef / 16lb per = ~71 Packers / 4 @a time = 18 Cooks!
When are you going to smoke this much? Where are you going to store 71 Briskets Raw and then Cooked, Brisket is not as good reheated as Fresh off the smoker.
What are you serving with it and where will that be stored? Do you have to serve it or just drop off hot pans of meat? Do you have Pots, Pans, Spoons, Sauce Despensers, Hot and Cold Holding equipment? Do you have Equipment to Prep and Store Raw and Cooked Ingredients? Do you have the required equipment to Clean and Sanitize all equipment including a Three Tub, Wash, Rinse and Sanitize Sink with the necessary Chemicals? Do you have the necessary Permits/Business License, Tax Id and Multi-Million Dollar Liability Insurance Policy? Does your State/County require all Cooking be done in a Health Inspected Commissary Kitchen, separate from your home, If so is ALL your Equipment Professional Grade and NSF Certified?
Different States have Different Requirements. Your Smoker, as nice as it is, would not be legal to use in all 50 states.
There is A LOT of work and equipment that goes into a 1500 person Catering Job. Above is just the basics to do this Legally and Safely.There is much I didn't go into. Lots to research and think about and pay for upfront. I wish you luck...JJ