Sorry if I missed this but who is responsible for the rentals (a perfect world is one in which the client owns all this but the question needs to be asked)
Linens, dishes, flatware, water and or wine glasses, coffee urn and cups, whatever you will be needing to pass or display apps plus everything you will need for the buffet line and or action stations.
To answer one of your original question...depending on your menu one person to help prep and cook (unless you are experienced and organized then you can most likely do this by your self 40 guests are not a lot), at least one person to keep an eye on the buffet (this person could pass the apps as well) and one to help clear tables and clean.
So you and one other person bare minimum.
Since this is an upscale event make sure whoever is pouring wine and clearing tables knows the proper etiquette.
For uniform, a white button down shirt (and tie of some sort) tucked into black trousers with a nice black apron or vest is nice as well.
You may already know all of this but thought I would toss it in just in case.