If i should start cake-decorating from scratch today, i would definately prioritize the shopping-list as follows:
2 smoothers - for smoothing the cake sides and top
3 different paste food colours, maybe red, blue, yellow
A dresden tool - indispensable for modelling
A small palette knife - excellent for lifting, cutting and shaping sugarcraft.
A balling tool - for rolling and cupping really thin flower petals
A foampad - useful for thinning petal and leaf edges on with the balling tool
A make-up brush - Useful to dust off the cake without leaving marks
Icing tips, -coupler and disposable bags - for Royal icing work
Small painting brushes
I would make my self a "duster" (foamwipes with maizena), and settle with my old wooden rolling pin and my very useful leaf-veiners right in the palm of my hand.
I would find a ruler, scissors, toothpicks, tweezers, straws and anything else useful around the house. Then i would wish for a toolbox for my birthday to save the little treasures in. A large one, cause they multiply quickly!
With just these few supplies and a good deal of imagination it´s possible to make beautiful cakes!
When you´re hooked on this hobby, and makes cakes for ANY occasion, timesavers could be:
A ribbon-cutter and/or a border tool
5-petal cutters and rose-leaf cutters in different sizes
Small rolling pins with or without pattern
...And then comes the rest! Molds, large rolling pin, impression-mats, edible glitters, sprays, other flower cutters, wires and tapes, kitchen aids, an even larger toolbox etc.
You'll never have to be in doubt when people ask what you want for Christmas anymore :)