Make sure you get a good skimmer rated at least double your tank size. I would avoid coralife brand skimmers that are often stocked on shelves. You will likely need to order one online which is fine time wise as you won't need it while tank cycles for ~4 weeks.
As for the water. The only way to tell if it's good is to invest in a TDS
(total dissolved solids) meter(about $10 on eBay) and test the water to make sure it is 0 TDS
prior to mixing salt in.
You don't need live sand. You can start with dry aragonite sand(cheaper) and it will become live.
Maybe check Craigslist for rock or start with dry base rock as live rock is often expensive. Though your tank is small enough that it won't cost much in live rock. Just pick out pieces you like. Generally a few bulky rock on the bottom as a base and less bulky pieces to build caves above that.
Don't forget to get testing kits. Liquid not strips.
Don't believe all advice from LFS
as they are out to make money. Better to ask for advice here.
And take your time.