Hi Sharon, any of Rose’s butter layer cakes would be suitable: red velvet, all occasion yellow downy, white velvet, chocolate fudge or all american chocolate butter cake would all work well. I wouldn’t recommend sponge type cakes or cakes that have chopped nuts etc in them ......the latter will make the shaping a little trickier. For the buttercream, Mousseline, classic, neoclassic and creamy dreamy would work well.
As far as the flavours are concerned. a combination that has a lot of impact when bitten into is the red velvet cake with the cream cheese buttercream (made with either white chocolate or powdered sugar) dipped in melted white wafers. This flavour also seems to appeal to a lot of people because it’s neither chocolate nor vanilla. For any other combinations, just keep in mind what the finished colour might be when the cake is mixed with the frosting….you don’t want to combine a vanilla cake with chocolate frosting (though you can do chocolate cake with vanilla buttercream)
Have you made them before? Bakerella has excellent instructions on shaping, freezing, dipping, decorating etc on her blog.
Good luck with this….it sounds like a lot of fun!