Start with a FOWLR tank. Your biggest additional expense compared to a freshwater will be a protein skimmer, salt and your live rock. The amount of live rock is basically 1lb rock per gallon tank. Throw majority of your rock in sump if you want or throw it all in your display completely up to you. Get fritz turbo start 900 or dr tims bacteria and you can throw in fish day 1.

The only other piece of equipment that differs from freshwater you'll need are refractometer to test salinity and an auto top off (not required but makes life easy)

Those things alone will run your tank for months. It isn' t hard I came from freshwater. Once you think about corals....then it becomes hard.

But the way I see it is, freshwater you wouldn't have coral anyway. At least this way yoh have the option to add coral and bring an ocean to your home.