Wedding Cake Dilemma

Hey guys, I have to make a cake for my relative’s wedding in August in Pennsylvania. The wedding will be in a barn and I am unsure if the barn has air conditioning or any type of kitchen and freezer units. My plan right now is to do a very small cake for the bride and groom and cupcakes for everyone else; But i still don’t want the cupcakes or small cake to be a soupy mess. Any tips on what type of icing or buttercream to use? I am an at home baker (and busy college student) and have only made one wedding cake before for an indoor fall wedding. I’ve also never used or made fondant. Any tips, tricks, and suggestions would be greatly appreciated. The cake must feed at minimum 125 people and they have not told me what flavor(s) they would like, they really haven’t given me any input whatsoever. So right now my main priority is the icing.
Thanks in advance!