A decent airbrush - such as a Paache double action airbrush, can be found for around $70 - $80 US, but you may have to look around some to find one at that price although I think Amazon has them. Make sure you get one with all 3 sizes of needles and tips. A #1 tip is for very fine work, the #3 is for gensral detail work and the #5 is for broad area spraying. A decent compressor can usually be found for about $100 - $120. Make sure that you get one with a small pressure tank. Do Not buy a "hobby" compressor because they will cause your paint to sputter due to the being a diaphram system. The tank is necessary to smooth out the air flow. Any other questions you may have about this I will be glad to help you all I can.
Sometimes an airbrush is a bit if over-kill when painting a Hull - unless you're building a plastic model. If your first coat doesn't look good a light sanding followed by a good cleaning off with either a tack cloth or a towel with a bit of alcohol on it will usually enable you to get a good second coat. Acrylics are notorious for not covering with one coat as a lot of them are not particularly pigment intensive. The cheaper the paint - the less pigment usually. If you have your heart set on an airbrush make sure you get a decent one. Stay away from Badger el-cheapo models.