Now i am making a marbled 8" cake filled with choc ganache and i was wondering whether i could then cover the cake in a buttercream then in marzipan so there is a very flat level surface under the fondant - the couple want buttercream as well as ganache but covered in white fondant with lace trimming....
I use Marzipan all the time as the final finish and underneath an (I)MBC frosting, so yes you can!
It does can get heavy/sweet with 2 layers of covering.. but I've heard it is more commonly done in the UK.
Refrigderated MBC can be made for a flat surface as well, but that takes some more technique. Ganache can even be made more flat.
Marzipan is usually used on heavy cakes, like fruit cakes, to even out the bumps from nuts, raisins, etc. before covering with fondant.
I personally don't think that it will go well with buttercream AND fondant AND ganache. I think the ratio of "covering" to cake will be too high and I think there will be too many icing flavors going on--the cake will get lost.
Proper smoothing of the BC will negate the need for the marzipan.
BlakesCakes is right. It will *work* but holy hell that's a lot of icings.