Hi Dan,
if you really don't want to spend too much to start with, a cheap .5mm gravity fed brush off eBay will do the job - it's a good way of finding out if airbrushing is for you, and then you can buy something better. Next step up is the airbrush kit Jon at RCS sells, they're good stuff, and plenty fine for spraying shells. The 'money no object' option for me is a Iwata; they cost a packet but are real quality items and suitably looked after will last a lifetime.
Air supply is also very important - you need a compressor that can generate at least 40-45 PSi continuously, and whilst a tank isn't a must have, it's definitely a real bonus!
Finally paint Fascolor is good (especially the flourescent colors), or Createx. AutoAir also works fine, but can be pricey.
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