Real Sword is really nice if you want an AK variant but as serveral have already stated not cheap. CA is probably the best for Armalites and possibly mp5's (though some might debate that). Various other brands, notably TM and Star, are good for other models. It all depends on what you want and how much you're willing to spend.
If $600 is your limit for airsoft period then maybe you might want to consider one of the midgrade guns, G&G, G&P or ICS. That will allow you to get some other gear as well, namely mags, ammo, battery and charger and a chest rig or vest. If $600 is simply all you're willing to spend until you make sure you want to continue with the sport then go with a higrade gun and then buy the rest if you like the sport. That's what I did, then I fell in love with the sport (and the guns) and I'm over $2000 now, got an m4, an m203, an ak47, a gp-25, an mp5k, an m9 and a 1911 so far.