I'd have to side with Bud, why not a few spray cans of paint?
IMHO:
If you have to ask how to get a smooth brushed finish, it won't happen
right away, especially on a car body. But a few tips - use a good
quality brush, keep the brush wet, long smooth strokes, and hopefully
the paint will level out. More than likely, followed by lot's of
careful wet sanding and rubbing out.
A cheap airbrush? Well, I would suggest an inexpensive AB but of good
guality. As far as an air supply, I would really consider other
alternatives to the canned air as they can effect your spraying.
I think what you should do is consider how much modeling you plan to
do. Start with some spray cans and get used to the feel of spraying.
Some very nice finishes can be accomplished with spray cans. Do some
searching here on the forum for such info as AB's and air supplies
(i.e. - canned air, compressors, CO2 tanks, air tanks, etc...). Armed
with that information you can decide on an AB which you can get for as
cheap as $40.00 and still be of good quality (I just bought one for
$20.00 on sale). If you can't afford a compressor or CO2 tank right
away, you could get one of those air tanks meant for filling tires.
That just means a trip to the corner gas station (or wherever has an
air pump/compressor) for refilling.
Just my