Looking again at the original picture, I suspect that it's actually been made in two layers; the white stripe around the bottom of the top deck is hiding the join. The top layer is on a thin board of its own, which is why the bits over the driver's window and rear doors are so straight.
Mumbug, if this is the first time you've worked with fondant I'd suggest NOT doing the recessed driver's window and rear door. Just do a tall rectangle with a curved top edge, cover the whole thing in red then add the details.
Honestly, I'm a really experienced cake decorator and I would quake at that recessed window
.
WRT putting a thin black layer underneath, I'd say you'd be making life much harder than you need to. You would need to stick the red icing to the black icing without getting any water in the places where you'd be cutting out windows, etc. Go with the cutting out and sticking black into the gaps, you'll save yourself so much stress!
Dowelling is another extra complication you probably don't need to worry about. Usually you use dowels to support the weight of each layer in a tiered cake which are all made on separate boards, so the cakes don't collapse into each other. It really won't make the bus any more stable. Just cut the base plinth 1 cm narrower all round than the bottom of the cake and make sure the cake sits centrally on it - it will be fine, I promise.
Re. freezing it. If I'm doing carved cakes I freeze them then carve them straight from the freezer because it's much, much easier to work with frozen sponge. You can do the filling, crumb coating and even the fondant before it's fully defrosted, it won't adversely affect it.
Time scale wise....it takes me a good 6+ hours to do a vehicle cake and I know what I'm doing
. Start early is all I can say!
::hazy memories of being up until 3am finishing a steam engine::
Good Luck! Shout if you need help...