I'll be at ExCel on Saturday!
If using a dummy tier, there's no need to buttercream. Lots of people just wet them all over to get the sugarpaste to stick. My personal preference is to use either piping gel or diluted liquid glucose.
If doing a really tall tier of cake, then depending on just how high it is, you'll need to put a board inbetween at either half way or two boards, each one a third of the way and then put dowels underneath, just like you would a normal tiered cake (only, in this case, the tiers are the same diameter, if you see what I mean?)
I always marzipan my cakes - even sponge cakes... it gives a nice smooth finish.
M&B do do nice colours, but not the best to work with. I use Regalice (or Renshaws Professional, as they've just re-branded it!!)