Dense cakes are easier to decorate than light cakes like sponges, though not impossible just more time consuming They also stand up easier under the weight of themselves.

Yep, most packet mixes are pretty foolproof. Just don't change the quantities of the wet ingredients at all and it will be fine.

If you want to just buy cake and not bake, to decorate any place that sells a decent mud cake type consistancy of cake is great if you want to do moulding (like a dino)

Or, this is really cheating you can hire some tins that already have that shape from most cake decorating places. A store called Cake Decorating Central comes to mind that does all sorts of tin hires.

Good luck