Fab cake .
If you are making a sponge cake then you don't need to use marzipan. Buttercream or warmed, sieved apricot jam is best.
The regalice ready made brown sugarpaste is supposedly chocolate flavoured but it's pretty vile, tbh. You can buy Chocopan from certain specialist cake shops which tastes just like proper chocolate. It's the same consistency as ordinary sugarpaste and you use it the same way, it's just a lot nicer to eat .
Top tips for getting your cake to look fab:
If you don't have much experience in using sugarpaste then I would do a round cake, iiwy. Getting the corners to go sit properly and not crack is very hard if you're a complete novice. (I've just done 5 square cakes and had to re do 2 of them because the corners were crap and I'm fairly experienced.)
You'll need a decent sized area of clear worktop to roll the sugarpaste out on as you'll need to roll it out to the diameter of the cake plus the height of the sides so a 10 inch cake that is 3 inches high will need a circle of icing measuring at least 17 inches across. Use plenty of icing sugar on the worktop to stop it sticking and move it around as you roll to make sure it's not stuck. Once it's the right size, lift it up using the rolling pin and drape it over the cake. Smooth across the top with the palms of your hands then over the edges and down the sides gently pulling the bottom out so you don't get any creases. Push it well into the crease between the bottom of the cake and the worktop then carefully trim away the excess with a plain, sharp knife. Don't worry if you get a few nicks as you cut, you can cover up any mistakes with the ribbon.
When you've made your cake, trim the risen bit off the top then turn it upside down. That will give you a completely flat top which looks much more professional than a lumpy surface.
If you're going to cover the board do it at least 24 hours ahead, more if you can so it can dry and harden and you won't end up with lots of dints in it if you knock it while you're decorating the cake. I would also leave the cake overnight before you add the smarties for the same reason.
Use jam or buttercream to stick the cake to the board so it doesn't disappear off the side when you carry it.
Once the cake is completely covered in sugarpaste you don't need to keep it in an airtight box, the sugarpaste does that for you.
Anything else you want to know, just ask!