Air Brushes are a good investment if you plan on painting a lot of large models and terrain. I would invest in a good compressor that allows you to adjust the PSI for the best results. If you decide to use the Propellant Cans I suggest you buy two, they begin to get cold during use and don't provide sufficient pressure, You will need good strong pressure when your ready to clean up when your done. So in the long run the Compressor will pay for it self. Propellant cans are 10-15 dollars a piece, you can get a good compressor around fifty dollars.
The best paint to use is Tamiya and Vallejo Air which designed specifically for air brushes. I use rubbing alcohol to thin the paint to the consistancy of milk, too thin is always better then too thick. I have used GW paints in the past, I thinned them with Windex which worked well you just need to clean the brush right away.
For clean up I use straight paint thinner, I buy a large jug at the hobby store and follow up with water to get out the thinner.
If your brush starts to have problems check and make sure the needle isn't bent thats usually the first thing that break/damage. If you get dried paint in the brush I would dismantle it and soak it in air brush cleaner which you can purchase at any hobby store that supplies air brushes. Hope this helps let me know if you need anything else.
Brian