Having come from freshwater myself I can tell you that there's quite a learning curve that comes with the complexities of salt. If you've cone freshwater planted tanks with sumps and external filtration then you've got a head start and will find it rather easy to move from fresh to salt. The main difference is the elements that are / aren't as important to test for between the two. Best piece of advice I can give is research, research, research. Start out by reading people's build logs from beginning to end, learn what mistakes they made, fixes they found, equipment lists, etc. This will help you get off on the right foot and help avoid some of the issues or problems that others experience.
Expect to spend roughly $30 a gallon for a "turn key" salt tank setup (not including livestock) if you get a used tank. Granted, that's a rough estimate and getting lower end equipment / used equipment will bring down that cost, just a good figure to estimate how big you can go for your budget. You don't want to get 90% into a 120 gallon and realize you don't have enough for lights or something along those lines.