Get a whole bunch of disposable decorating bags and disposable plastic tips. These bags are clear so people can see what colour is inside, and everything can be cleaned up quickly at the end.
For spreading, use plastic knives (picnic cutlery) and small disposable plastic tubs.
Be specific about how many cupcakes each person gets, and price accordingly. It might even help to provide boxes for 6 cupcakes if that is what the price includes...just to make sure you have enough for everybody.
A simple printed instruction sheet would be nice.