I will say this about cheap vs. expensive. It *can* be worth it to go expensive. I don't know where Badger airbrushes stack up these days, but I know when I bought mine probably 15 years ago now, to replace a cheapie Testors brush that had died on me, Badger was a good one. All metal, fully dismantleable, etc etc. And the thing is still in very good condition, and works FAR better than the cheap Testors brush ever did. So keep that in mind. Of course, I also knew I'd be using it for model airplane paint jobs...alot...so I knew going in it'd be worth it.
Also, if you want to do actual artwork on your cases or anything else, consider taking art/drawing classes if you haven't already. Some of brush/pencil/airbrush skill is technique, but a large amount is training and developing eye-hand coordination so you can evaluate what your eyes are telling you and translate it from 3 dimensions to 2. Drawing from your imagination is a little different, but good artistic training is invaluable if you want to get really good.
That being said, yes I have artistic training, but I lack the desire to draw except when I need to (design drawings and such), so I never got all that good at it.