Freshwater to Saltwater Switch plus other questions?

I have a 30 gallon freshwater tank right now with a stand built for 40 gallons, a heater, a good filter, a kind of pebble gravel, and one large centerpiece rock, and one small rock for the side. What else do I need to convert into a saltwater tank? What can I use? What will be the final price (without fish)? I am not planning on buying coral, anenome, etc. What are some fish I can put after I cycle my tank and all that other stuff? Thanks to anyone who answers everything.