I have a few questions about filters.

I am going to set up a 75 gallon tank for salt water. I know what my options are for filtration. Just wanted to know the pro's and con's of them. I own a fluval 404 canister, aquaclear 500 over the back, and a under gravel. I was hoping to get some clairity on what many of you recomend. I am considering purchasing a wet/dry. But if i do i want to keep it as low cost as possible. So how do i check if that it will be adequate or not. I have noticed that ebay has complete setups (wet/dry) reasonably priced. Are they good or garbage. And in a wet/dry what keeps water from draining out of the sump on to the floor? To be completely honest i want to use as less filters as possible to keep it as quiet as possible. Thank you for takeing the time to read this and helping me!!!