@lironp, a few thoughts on your decision: I have used 4 different airbrushes, and by far the best has been the Grex. I find the gun-like trigger action easier and more comfortable to use than the more prevalent ones with the trigger on top of the gun. The Grex is more expensive than many other airbrushes, especially if you upgrade to the .7mm nozzle, which works better for cocoa butter, and it requires more compressed air and so a stronger compressor. If you keep it warm (which I consider a "must"), it sprays beautifully. I bought a dehydrator to keep it warm, and the model I purchased is great. The Grex has a larger size cup, and that is a crucial consideration; stopping to fill up the cup as you spray is a major nuisance.
HVLP spray guns are, of course, similar but are different in many ways. I bought a very inexpensive one from Harbor Freight tools. It sprays more rapidly than an airbrush but uses more cocoa butter. Mine does not do splattering, but some of them do. Here is a link to a Grizzly model recommended by @Bentley, and he stated that it does splatter very successfully. Of course, an alternative for splattering is to use toothbrush, recommended by some famous people in the field (such as Andrey Dubovik). I also have a Fuji spray system, another version of an HVLP gun. It works very fast but uses a huge amount of cocoa butter. From what you wrote, it is beyond the price range you are looking at.
As people have posted in this thread (see especially the comments by @teonzo), the compressor is a crucial choice. It's not just the HP but the amount of storage of air that counts. My Grex works fine with my 2HP, 2-tank California Air Tools compressor, but the El Cheapo HVLP gun kept the compressor running almost constantly to provide enough air at high enough a pressure. If you go with an HVLP gun (such as the Grizzly), I would recommend a compressor larger than what I purchased.
So if you will be doing 10-20 molds, an argument could be made for either choice. An HVLP gun will use more cocoa butter and will get them done faster than an airbrush, and if you want to do splattering with whatever you choose, the Grex does not do splatter, and only certain HVLP guns do. No matter what choice you make, I would strongly recommend getting something to keep the cocoa butter and gun warm. My dehydrator has changed my life (as far as decorating molds, goes!). And, sorry to say, you might be able to stay within your desired budget, but it will be difficult, and (in my opinion) allowing for expansion (making more chocolates in the future) will save you in the long run. I predict that as you get into more elaborate decorating of molds, you will be hooked, like so many of us on eGullet.