Testors makes good air brushes for the price. You can usually get a kit with 3-4 bottles of different sizes and 2-3 nozzles for about $100.
If you want to get more artistic (as in fades and highlights etc...) your going to want a dual action to control both the amount of air and paint to be released. Its not hard once you get the hang of it.
If you plan on doing many bodies (at least 2-3 a season) you may want to invest in a compressor or an air tank. Again, for about $100 you can get a 5-10Gal portable tank that just needs to be taken down to your local gas station to be refilled. A compressor/airtank combo may be more $$ but then you dont have to lug the big tank around to refill. No mater what source of air you choose dont be pennywise and pound foolish. All air tanks will require a regulator and a moisture trap. Dont skimp on these items. I've seen many a paintjob be ruined by moisture.... Then there are the propelland cans. They dont need a regulator or trap and go for about $0.95/ounce. You will need at least a 9oz can to do your average body.
If your not going to be doing that many bodies or your not ready to get too artistic...just stick to rattle cans. With some time, masking tape, a little effort, and patience you can have a really good looking body wh\ithout spending more than $5-10 a color.
Just my $0.02