First of all, don't consider any question that you don't have the answer for "a stupid question". That especially applies with fish and sw
tanks in particular.
Definitely top off evaporated water with fresh water. Use the sw
for water changes and the initial fill-up.
One of my local grocery stores has a water unit outside that sells ro
water for 25 cents per gallon.
See if your lfs
will lend you the 5 gallon jugs that you need for initial fill-up.
Invest in a 5 gallon jug or two to have for transports after that for water changes.
You can mix your initial water in the tank. In regards to the timing of adding the sand, I would suggest mixing the water first and then adding the sand slowly after all the water has been mixed.