If this is your first cook, I would think doing pulled pork and pulled chicken is a better route than ribs, unless you have a large enough cooker to pull it off. A lot easier as you can do a lot of prep the day before, which means less effort when your clients are there.
The cooker and equipment is pretty important to really answer your questions. Four people for a 100 person even seems a little light if one or two of you want to spin off and schmooze. Also, what about beverages, utensils etc...
me: I don't drink anymore
Yelonutz: me either, but, then again, I don't drink any less