If the wedding cake is for 125, for 300 people I would consider at least 200 cupcakes- maybe 72 (6 dozen) of each? People do take one of several flavors.

Fancy cupcakes aren't cheap. You might consider a grooms cake, chocolate or spice, for part of the cake supply.

With cookies and brownies, I suggest 1 of EACH kind per person up to 4, but they should make small ones- 2 to 3 bites. People will take several different ones. At that time of year, an assortment of Christmas cookies and bars is a delight- and they can be made ahead and frozen.

This choice of beverages is a more difficult estimate, as it depends in part on the ages of the guests.

coffee, use the beverage planning page, reception level, and do have the suggested tea and decaf as part of it.
cocoa, maybe 10 gallons, consider putting some ground ginger and cinnamon in; can be dispensed in a coffee urn; awfully nice to have cans of real whipped cream to top, and maybe peppermint sticks (the skinny ones) to stir.
cider, maybe 8 gallons, plenty of spice, don't forget the clove; can also be served from coffee urns.

With 300 people, you want at least 3 beverage dispensing stations.