A canister has its place but in the right hands. But not needed and can actually add to poor water quality. In a salt water tank your LR and LS are your best bio filter.
For a new setup you need a quality water source and powerheads for water flow and LR and LS at a minimum of 1 lb per gallon. Though you dont need all LR you can get a little LR and then add base rock which will become live. Then a skimmer and lights. Then your tank needs 4 to 8 weeks to cycle. I would get a book and research the whole setup and cost before you start.
A 125 with the basic equipment will increase your monthly electric bill 30 to 60 bucks alone then there the salt and water that add up. So you could be looking at around 100 bucks a month just to maintain its normal requirments. A book would be a great start. GL..
Amazon.com: The New Marine Aquarium: Step-By-Step Setup & Stocking Guide: Michael S. Paletta, Edward Kadunc, Scott W. Michael, John Goodman: Books