Good on you

You can make the cake well in advance (with most recipes) and freeze then thaw before decorating if you really want, but I don't think they ever taste as good, personally. I don't freeze them - depending on what type of cake you bake will just depend on how long in advance you can make it. Buttercream does have a tendency to sweat and the colours can bleed if you store it in the fridge (as it warms back up again) - I usually just decorate the day before and then store in a box in a cool place.

For decoration bits, I usually make marshmallow fondant but it depends how confident you are - it's a messy process, but one that I think is worth it - it tastes so much better than normal fondant. It's pretty easy, tho, just time consuming. Then you just roll it out and cut it as needed.