One drawback with using an overflow/wet and dry system for freshwater is that it's not great for the plants, due to the CO2 off-gassing. That's not to say you can't do it, but it is a factor. We have a planted 72 gallon bowfront that was once a saltwater FOWLR.
Most things about a saltwater tank are a lot more expensive, including equipment, lighting, test kits, salt, etc. The fish are probably the least of the expense. For something around 125 gallons, it will probably add up to a couple of thousand over the first year for setup, etc.