Best grab a drink and sit down. I "do" go on! 
What you don't want is an expensive airbrush, that ends up sitting in a box because you aren't comfortable using it. Or one that is hard to get parts for. It is very easy to damage the needle, and if you don't have a spare and can only get them online, you're dead in the water until the part arrives. See if anyone stock parts for airbrushes in your home town, and buy the best you can afford of that brand.
The BIg Three are Badger, Iwata & Paasche. Any of those (and more) are available at sites like
http://www.bearair.com/
http://www.dixieart.com/
http://www.tcpglobal.com/airbrushdepot/
Those are just three sources, there are many, many more!
There are also brands like Peak, RichPen, Tamiya, Efbe, Thayer & Chandler (owned by Badger), DeVilbiss, Harder & Steenbeck and Aztek (owned by Testors). They all have their adherents and are all useful. Personally I stick with Badger, as they are US owned and made, and their service is extraordinary.
AIRBRUSHES:
Airbrushes come in different configurations, and you'll have to choose for yourself which one would be best for you. Basically you have to choose between internal or external mis, single or dual action & siphon or gravity fed.
Internal vs. External Mix:
Internal mix air brushes atomize the paint into the air stream within the body of the airbrush, while external mix do so well, externally.
External mix are very easy to clean, but do not offer the range of control internal mix do. My preference, and that of most people is internal mix. Examples of internal mix airbrushes are the Badger 100 series, Iwata HP-CS Eclipse and the Paasche VL. External mix include the Badger 350, and Paasche H
Single vs. Dual Action:
Single action has a simple trigger: air flow on or air flow off. You control the line width by adjusting the needle postion and that position is "locked on" until you reset it. Very simple to use, and very precise. I have a Badger 200-20 "detail" brush that I use when I need extremely fine lines. I can set it and forget it, and the width stays consistent, even after cleaning. Popular single action airbrushes are the Badger 200 series, Iwata SAR, or the Paasche H
Dual action triggers control both the the air flow by pushing down on the trigger, and the needle position by pulling back on the trigger. Sounds harder to use in theory, but its pretty easy to use in practice. Dual action allows you to alter "on the fly" between thin and thick lines. Examples of dual action are Badger 105 Patriot, Iwata HP-CS Eclipse and Paasche VL.
Siphon vs. Gravity Fed:
Siphon fed brushes use either a small bottle or a color cup that attaches to the bottom of the air brush body. They require sufficient air pressure to pull the paint up into the air flow. Great for large surface area painting, they require more attention when cleaning as you have to deal with the paint left in the siphon tube. Examples are Badger 155 Anthem, Iwata BCR Revolution, or the Paasche VL.
Gravity fed have a permamently attached cup mounted to the top of the body. This allows for slightly less air pressure as gravity takes care of delivering the paint into the air flow. Less air pressure can result in finer spray control, or I should say finer OVER spray control. Relatively easy to clean, and slightly less wasteful of paint, as there is no paint left in a siphon tube. The Badger 100LG, Iwata HP-CS and Paasche Talon are good examples.
There is a third feed mechanism, though not often used in modeling, known as side fed. This has a cup that mounts to either the left or right (depending on the handedness of the owner), and is primarily an illustration air brush. It works much like a gravity fed air brush, and the cup can be rotated so the user can use it pointing straight down. Examples are the Badger 100SF & the Iwata Eclipse HP-SBS.
AIR SUPPLY:
Perhaps even more important, and which can easily can end up being the more expensive aquisition, is a good compressor or other reliable air supply. Your choices are propellent cans, propellent tanks, hobby compressors, air brush compressors or utility compressors.
Propellent Cans:
Often the first choice of beginners due to their intial low cost, some air brushes even are sold as a kit with the air brush and can in one package. I say initlal low cost, because you never stop paying for them. They can also be frustrating to use, as they don't hold that much air. And they have a habit of running out of air before you run out of the requirement for air! You also have to consider just how much garbage they create, as the cans are not refillable. You can buy an entry level compressor for the cost of about a dozen cans. Their main benefit is quietness. Available from your LHS, or online model stores.
Propellent Tanks:
These refillable tanks are typically used for supplying CO2 for soft drinks, or as a supply of nitrogen for welders, these tanks offer a much greater volume of air (though I suspect they still run out at the worst time!) and are completely silent, other than the hiss of the air brush in use. Both gases are good for air brushing as they are considered "dry", in other words, the compression of CO2 does not squeeze water out, as will happen with compressed air. They are fairly large, and can be a hassle to refill in some communities. They also have a fairly high startup cost. Since you have to pay for refilling, I don't consider them a good investment. On the other hand, they will allow you to paint when there are sleeping babies in the house. It all depends on your unique situation! Available direct from the gas supply company in your area.
Hobby Compressors:
These are small compressors, typically a diaphragm powered supply. They can be found for as little as $50 at places like Harbor Freight, or as a "brand name" for around $120. Fairly noisy, and low powered (<40 psi), they can burn out quickly. I don't recommend them except for the severely budget restrained (I'm married, too!). Typically offered only as tankless models. Refer to the online stores recommend above.
Air Brush Compressors:
I consider these a step up from the hobby compressor above. Basically the same compressor, only on steroids. Prices can range from $150 to YIKES! These are the types used by air brush artists, nail salons and t-shirt artists. Capable of running for long periods of time, and offer a better range of air pressure. TCP Global have some interesting models, as do Paasche, Badger and Iwata. They are typically paired with a small tank. I recommend double piston tanked models. Refer to the online stores recommend above.
Utility Compressors:
Typically found in garages, they are usually have tanks ranging from 1 gallon and up. I have a friend who has an automotive garage business at home and he hooks up to a 100 gallon behemoth. He NEVER runs out of air! These compressors are invariably loud, and large. However, they do allow you to fill your tires and run pneumatic tools. I use a 3 gallon "pancake" style and run a hose to my spray booth. Any reputable hardware store is a good place to buy these.
EXTRA SUPPLIES:
Propellent tanks and compressors require an air hose, and compressors should also use a water trap and particle trap (often combined). I strongly recommend an air regulator, to insure a consistant pressure. A good respirator is strongly suggested, as the fumes and particles from any type of paint is very hard on lungs. Various fittings to hook the hose to the compressor and also to the airbrush can be necessary, depending on your situation. All joinst should be reinforced with teflon tape, availabe in the plumbing section of your favorite DYI store. A spray booth is a good idea, but can be expensive for beginners to justify.
Still here? Sometimes I wish I got rated on word count, and not post count! 