You'll get a lot of different viewpoints on brushes. Probably as many as what car is the best to run. I have had GREAT success with a Paasche VL set, since I started playing around with brushes in 1983. The first brush I ever had was a Badger single-action brush, and for screwing around, it was perfect. Once I figured out what I was doing with it, though, I quickly recognized the need for more control (and I was only 13 at the time).
Through college, I used a high-end Thayer & Chandler dual-action gravity feed for paintings and other fine detail work, but for R/C paints, it was too fine and couldn't shoot various viscosities very well. I still use it from time to time, but only for very detailed work.
The cost of a VL set is around $60 and it comes with 3 tips/needles, hose, a siphon-feed cup and a few bottles and lids that are the exact same as the original Pactra toxic RC paints. For what you get, it's a fantastic price, and the brush is an absolute workhorse, as long as you keep it clean and maintained.
I have seen somewhere that someone sells an adapter to screw onto Parma Fastcolor bottles directly, but I forgot where I saw it. I know there is one out there for SpazStix bottles, too, I think.
Almost as important as the airbrush is the air source. You can work from pressurized cans of air, tanks of air and even spare tires with an adapter on the valve stem, but nothing as good as a compressor. There are small, silent ones out there specifically for airbrushing, but they never suited my needs very well. I prefer a small "pancake" type compressor that you can use in the garage on other stuff, and still offers an air holding tank, adjustable PSI and drainability. I just picked up a new pump at a local hardware/tool store for $169 and it will power a impact tools when my workload focuses on real cars.
If you want show finishes for plastic models, plus the flexibility of using all of the cool colors offered for RC cars, rattle cans just aren't gonna cut it. You'll be happy if you buy an airbrush and spend some time practicing with it.
Start saving all of those clear pie and salad containers you get from the grocery store, they make great practice "bodies." A few masking experiments, and a lot of cardboard for learning how the gun works in your hand, and you'll be painting like a madman in no time.
-doug