While the copper won't matter in a fish only system, if your tank has had copper in it, you will not have the ability to switch to anything but a fish only system in the future. not that you should not make the jump to salt just because of this, but rather that you should just know for the time that you'll have had the fish only system for a year or two and you want to add a coral or something - having once had copper in it, such a tank just cannot house corals.
as you can see from the advice you've gotten already, there are LOTS of ways to do a tank. While some would do it one way, others would never do it that way. There is usually no one BEST way to set up your new system. Best advice is do a lot of research, have a good plan - optimize your chances of success and keep the most options open for yourself for the future.
That said, increasing volume is always a good idea, but a 120 gal tank is certainly a lot of volume already. A sump is always great because you can tinker later on - add gadgets and other stuff - but what you do will partly be regulated by your wallet, so do the best you can and jump into the water when you're ready! Keep the questions coming and Good Luck!