First post for first SW Tank!

Morning Y'all!! Like many FW enthusiast I have been looking into the SW field. i have been looking into it for almost a year now, lol yes a year and been very hesitant because I want to find the right tank, right equipment and the right prices of course. Been to my local stores and they have suggested Coralife Biocube. I love the look and the idea of it all. Ive read different posts about the aquariums and understand having to do live rock in the back, change some lighting different pumps. I am planning on grabbing a Coralife either 14/32 still not sure, just was curious if I did the 32, how often and how much water change would I need per change? Also I am planning on buying the remade salt water from my reef store in the area and was curious if I purchased it by the gallons how long would it stay "fresh" for in proper storage? THANKS!