You should never go into saltwater thinking its not going to be an investment, no matter the size tank.
I agree with TOS. You'd be better off buying a light fixture with some sort of daylight and some sort of actinic lighting. How many watts is this light you're using now? You may not be able to keep any coral under it.
Something like this would be okay for some of the easier soft corals, but it has low wattage even for a 5 gallon tank.
http://www.drsfostersmith.com/produc...2&pcatid=11382
If you are handy, you may want to look into setting up a small retro-fit light of T5s or CF.
If you're going to go reef, there is almost no point in going with one coral. The point of a reef is to have a variety. A zoa tank would look really awesome, but having one zoa is going to be like having one dime-sized piece of coral in a whole tank.
This is an example of a 2 gallon pico reef that looks amazing, and is actually incredibly simply set up.
http://www.melevsreef.com/pico.html
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Current setups: 1800 gallon koi pond, 10 gallon planted, 150 gallon reef, other FOWLR tanks