So I'm trying to attept to make a cake with a base made out of two 13" x 9" and would like to make 2 semi truck and trailers for the top out of rkt ( Rice Krispie treats) for my Husbands surprise 30th bday party. This is my first time attempting something like this.

Wondering which fondant would be best to use for cover the rkt truck and trailer, Rolled buttercrem fondant or Marshmallow fondant?

which one of these would also be better for making all the details such as bumper, lights, windows etc that would hold its shape?

Also how far in advance should I start making? How to store when completed? Was thinking of putting in our garage which is a rec room to keep it from him seeing but it is still abit cold in there, would this harm it? Will the cold be okay for it?

Advice pleassssse! Thanks you