For general rocket painting, I'm going to recommend the tried and true Paasche H. Since a lot of rocket painting involves covering large areas, getting one with the #5 (large) nozzle is very handy. They make a kit that comes with three tips (#1, #3 and the all important #5) three jars, hose and miscellaneous other bits for ~$40 on eBay.
Yes - you can find cheap testors/aztek kits complete with a can of compressed air at places like Walmart. Do yourself a big favor and don't get this setup. The nozzles are not internally cleanable and therefore if you ever clog one up badly, you'll need to buy a replacement nozzle. Not fun.
The air source will be your other major expense. Ideally a compressor and is best, but that's the most expensive in the short run. Lot's of people use "alternative" sources like the air in a spare car tire or a CO2 tank. One option that isn't too expensive ($20-$30 at Sears) is 5-gallon air tank you can fill at your local gas station. You'll still need a regulator - under $10 at Harbor Freight, or maybe up to $20 at sears/home depot. Maybe Harbor Freight has air tanks, too.
Harborfrieght has a $10 single-action airbrush that looks to be a fairly good knockoff of the paasche H (or Badger equivalent). It reportedly works reasonably well and as it can be dissasembled for cleaning; it has got to be better than the cheapie testors kit at Walmart.
I do recommend single-action over double action for rocket work. It's simpler, easier to clean and I think more tolerant of a wider variety of paints. Not a lot of rocket work involves fineline detail work, which is where double-action ab's shine.