This is the one that I started with in 1985. Oddly enough, what it's going for today ($60) is about what I paid for mine when I bought it new in 1985 at the graphics arts supply shop, and the 'used' ones on ebay sell for about 90% of new, so if it's not for you, it's a quick flip and get your money back out of it!
Click here for Paashce H @ Amazon Try flipping a budget airbrush....you'll be lucky to get 50% of what you paid for it.
As to the issue of air supply, a moisture trap of some nature is 100% necessary. Same with pressure regulation. Tankless airbrush compressors, as well as home air tool pancake compressors, are prone to transmitting the compression pulses from the piston to the tank, causing micro fluctuations in the air supply, even when pressure regulated. Since it only happens when the piston is actually running, most don't see it much on the home pancake compressors, or maybe don't realize what they're seeing in the spray pattern. The larger the tank, generally, the more stable the air supply. For basic blocking in airbrush work on models and rockets, it won't matter very much.
To the issue of internal vs. external mix, internal will give a slightly better spray pattern, but external is easier to clean and maintain. Single action vs. double action most folks have virtually zero need for the complexity (and added expense) of double action (both in airbrush and skill set) unless you're into graphic arts and are using it on your rockets.