Bikerbuster,
probably, when you’re reading external and internal mix, you’re also seeing single or double action. Action refers to control, in this case.
An external mix, single action, allows you specific control on air. Push the air button and paint is spraying. You will have some control, though limited, on the volume of paint (which ultimately controls how fine a line and how much paint is applied to the area) you achieve with a single action airbrush. These airbrushes are very useful for large areas of coverage or if you’re really good at masking. I have one that I use for clear coats, simply because I’m going for large area coverage with a wide mist. This being said, I know a lot of people consider them a good starting point, and generally they can be found fairly cheap (mine cost $30 cdn).
An internal mix, or double action, gives you control on air and paint volume. Push the button, air is moving, pull the trigger and push the button, paint is spraying. Pull the trigger further, more paint, less trigger, less paint. Here you can get a fine line, you can lay down light layers, you can do a wide mist. I prefer a double action or internal mix because I get the best of both. I find a double action offers me more control in fine lines, medium and wide coverage. They are a little bit more to learn, you have to train your trigger finger, they range in price quite a lot (my cheap one cost me $120 cdn, my expensive set $410 cdn).
At first, I used propellant cans. At $14 a pop, you’ll pay for a compressor in no time.
So, considering an air compressor, you will want a moisture trap (you should have one) and a pressure reducing valve with a gauge. Some compressors come equipped with these, others don’t. Mine didn’t. I looked online at hobby sites, shopping for a moisture trap and a valve. Ultimately, I went to Canadian Tire (a generic hardware store) and found a much cheaper set up than I could have online, and I accomplish the same job. It all depends on what you wish to spend.
My compressor cost around $100 cdn, the moisture trap around $15, the valve was about $8. Hobby stores online were much more expensive, I think a combination moisture trap and pressure reducer that I was looking at was $90 us.
After all of that, you will have to figure out which base of paints you’re going to use (acrylic, enamel, lacquer), thinners, thinning ratios and air pressure. It is a bit of a learning curve, but the results can be gorgeous. You will achieve much more than you ever could with a paint brush.
I know this is a little long winded, but I hope there’s some useful information in there that helps with your decision.