What is your main course going to consist of? I guess the main "X-Factor" here would be your particular market. Around here, people love spanish rice & beans so much that it's usually served as an appetizer while the BBQ is finishing up on the grill/smoker.
I almost always throw a pot of beans (but not rice) in the smoker when I fire it up. I usually figure about 2# of dry beans per 10 people. That works out to 1/5# per person BUT I always shoot to have leftover beans to stock in the fridge/freezer for later use.
For a tentative 400 guests I think you could get away with 50# of dry pinto beans. I'm imagining one of those styrofoam to-go boxes with the 3 compartments - one large and two small. The max ammt of finished beans that'll fit in the small compartment is 3/4 of a cup. Rice would be about the same - maybe 1/2 cup packed. That works out to approx 1/4 cup dry beans per person. So one cup of dry pinto beans you buy will serve approx 4 people. My fuzzy math is telling me that you'd need 100 cups of uncooked beans -IF- all 400 guests were to eat beans. I estimate that there are 4 cups in a 2# bag so figure 2 cups per pound of uncooked beans. Soooooo..............for 100 cups of uncooked beans you'd need roughly 50# of uncooked beans.
Wow, the 50# estimate was just that - an estimate and after doing the math, it worked out LOL.
Now, to quote Austin Powers after doing the math, "Oh no, I've gone cross-eyed!"
I hope that helps you out some,