To be honest, when I was taking graphic arts classes my instructors told us all the time to use sketches to start with, i.e. pencil & paper, or colored pencils if you chose. Not sure if you're looking to have the Junior painted, or wrapped. But, in either case you don't necessarily have to "draw" it on a computer, or spend the $$$ to purchase the software in order to create a paint scheme. Not to mention having a computer capable of running the software. I know that pencil & paper are back to basics. But, if you've never used Photoshop, or Illustrator, sometimes it can be a tough learning curve. Not trying to discourage anyone, just saying there was pencil & paper before there was Adobe.
Personally, I don't think you need a 2D/3D model to design a paint scheme. Sure it looks cool, but a model is a bit overkill just to design a scheme. Not to mention modelling software can be more expensive than Adobe products.
Whichever route you decide, just remember to have fun with it. Just curious, but what's your time frame on this?