perryfish, first of all when tap water, or any water evaporates, it is only the water that goes, all of the bad stuff stays in the tank. All water evaporates the same.
I think RO water is just about as good as RO/DI but get a second opinion
Southdown sand is dry playsand, not live sand, and it is a lot cheaper to buy this way. If you seed it with a little live sand it will turn into live sand. I put 100 lbs of it in my tank along woth 80 lbs of livesand. You could go for even less LS than I did though. the only bad thing is that it will cloud a lot. You could do what I did in my brothers tank and put down the southdown
D ) and then put a layer of LS on top and then put the water in so the southdown is at the bottom and won't cloud. I found my southdown at Home Depot.
p.s. What are you going to be putting in this tank?