I would recommend using a good quality salt and doing weekly water changes. I was spending a fortune on supplements before doing this. When I started ZEOvit, I had to do weekly water changes and I have noticed I literally need half of the dosing I used to in order to keep levels up. Just 5G a week for me. Not a big deal. Plus it feels really good to know I am "cleaning" the tank each week.
Regardless of weekly water changes or not though, I think a good salt is worth it. Why skimp on something so important? Once you get Mg levels up, they do not get used up quickly and if your salt is good, just water changes will be enough (as long as you do at least one a month).
Using Instant Ocean has never made sense to me...you have to add so much to it and your still not getting a lot of trace elements. I know my tanks looked 10 times better after switching from IO to Tropic Marin Pro salt last year. I have now had to switch again because J&L does not carry Tropic Marin anymore but I am finding the Deltec natural sea salt to be very nice to use. I have tested it each water change and levels are perfect each time.
EDIT: Hmmm...maybe you said you don't really have sps. If not, then nevermind
But if you do get into a lot of sps (which you plan to), I would go with a top quality salt.