I'm not trying to scare you away, but just to let you know what a friend of mine recently went through...
He had a 20 gallon pirahna tank, decided to convert it to saltwater
30 pounds live rock - $450
compressed lighting - $200
Argonite sand - $50
Salt, for the water - $50
fish and corals - $300
This setup lasted for around 6 months, basically, I'd do his water changes when I came over, he didn't realize how often he'd need to do them on a 20 gallon without a sump or anything. Two weeks ago, this tank and everything in it, was sitting on his curb. garbage. he gave up on it.
Just basically wanted to let you know, make sure you know exactly what you need, before you even try.