keep in mind, one of the largest hidden costs for a tank is the stand and canopy, you can easily spend 300 dollars just on that!
For less that expensive gear that works on a system abou that size look at my sig. The only thing i would do is figure out ahead of time what kind of invertibrates you want to have, because lights can be really expensive, and you don't want to have to buy them twice. If you are going to keep primarily fish, and will be staying away from things like clams, corals, and anenomes (which is near impossible on a 300 dollar budget) you can invest in about 110 watts of power compacts for around 100 dollars online, get a 80 dollar protien skimmer, and spend 20 bucks on a pwerhead. Live rock will average about 6 dollars a pound in most places for basic, non coral encrusted stuff, so keep that in mind. try to have no less than 1 pound per gallon, especially if you are going to go filterless.
To save money, if all you will have is fish for the time being, you can get a single bulb, normal output flourescent and a bulb for it for about 40 dollars, which will save money over the power compacts. Also, if you are going to keep yourself to just a couple fish to start, instead of a skimmer, you could go with a hang on tank power filter, a magnum, or an aquaclear. this route will cut donw a lot on the need for as much live rock.
Bare bones, cheapest i would go, 150 watt visi-therm or ebo jagger heater, a sturdy stand, a glass top, a strip light (preferably 2 bulbs if you can fit it in the budget), a 300 or 500 series aquaclear filter, a good test kit (about 30 bucks for a basic one), a small powerhead for water movement (rio 90 at 15 dollars would work), and about 15 pounds of live rock.
Upgrades would be about 30 pounds more live rock, a canopy with PC lights, a skimmer to replace the filter (after live rock purchase), a larger powerhead, or a second of similar size. This configuration would put you in the 500 to 600 dollar range, and most of these could be done in steps (remember, you need lots of live rock to replace a power filter, and then a skimmer is necessary, so they go hand in hand).
If I was you, I would probably recomend saving another hundred dollars or so before setting up your tank, because that will give you a lot better shot at keeping things alive, especially if you hope to keep more than just that hawkfish.
On a side note, check your newspaper classifides, there are frequently salt water tank setups in the paper at drastic discounts, just make sure to see that everything is in working order, find out how old the bulbs are, if the tank leaks, etc..... generally ask why its being sold. if the person is getting out of the hobby, or their tank crashed and made them mad, its probably a safe bet that the stuff is ok, if its because everything is old, i wouldnt buy the stuff. just a thought. if you want more information on buying used equipment, ask that in another thread
Good starting fish would be a couple of green or blue chromis, maybe a blue damsel, a ocellaris clown, or a firefish, how many you can have will be dependant on you filtration. Also you will want one hermit and one snail per 2 or 3 gallons of water, so 15 or so of each to keep algae under control.
