I would agree with Seven. If you are considering adding corals to a fish only tank I would start with softies. Two reasons, as Seven mentioned they are typically less costly than SPS or LPS stony type corals. They are also easier to care for and a little more forgiving in water quality than the other types. Not only that, if you for whatever reason have a tank crash or die off you will minimze your losses as far as cost goes.
You will want to check your water parameters before adding any corals. You may also if you dont already have some, consider going with adding some fully cured live rock. This does a couple of things it looks nice but more importantly it is a great bilogical filter and will compliment your wet dry. The rule of thumb is 1- 1 1/4 lbs of rock per gallon of water.
My question is this.... are you planning on just doing a few softies and stop there? The reason I ask is if you are planning on doing more than this you are going to have to start doing some reasearch and homework on reef keeping. The mere fact that you are here asking questions is a good sign. I personally would recomend even for softies that you upgrade to VHO, T5 or similar lighting. Especially if you are only using regular output flourescent lights. I am sure most people here will agree.
Do not let your LFS tell you you have to add costly metal halide lighting for softies this is just not true. If at such time you want to go with LPS and SPS type corals you will need to have more intense lighting but again you dont have to go MH if you dont have the cash. I have seen many wonderful stony coral tanks with no metal halides at all. Just good husbandry and a lot of VHO lighting can keep stiny corals. Although you will find that the opinions on this are as varied as the stars.
Again the single best thing you can do is get a good book on marine/reef tanks and do some reading.
The filtration you are using is probably going to pose some issues concerning nitrates, there is much debate over the use of bio balls in a coral tank. I would not use a wet dry system with bio balls for a coral tank. But if you are going to stay fish dominate and add just a few softies then maybe you can have some success.
I am hoping others will pipe in as I am not a reef guru by any stretch, but I have this time around, done a lot of reading and listening to what others have done to keep corals.
A word of caution... if you are like the rest of us you will become quickly addicted to having and keeping corals and the urge to add more will probably come. Go slow, read a lot, ask lots of questions and remember that you and you alone are responsible for doing the right things to assure that your critters have a healthy good life in your tank. I am a little concerned with the bio load you already have with that many fish. I believe there is a formula (not written in stone) about how many fish per gallon of water you can use as a guideline for your tank.
In closing the first thing that caught my eye in your post was asking about a cheap way to go about things. I can personally attest that there is nothing cheap about the hobby of keeping corals. Unless you are an avid DIY er and can fab a lot of your own stuff.
Well hope this helps some. And pardon me for the lecture about "you being responsible speech about your critters health". I am just an avid supporter of saving the reefs as we know them, each one of us can do our part by becoming as knowledgable in keeping marine tanks because if we are successful in keeping livestock then hopefully we can limit the demand for the constant harvesting of live/wild caught specimens. I will now get off my soapbox.
