I started my tank with tap water, and quickly started doing water changes with R/O. If you have naturally soft tap water which is free of nitrates and phosphates, it could be used in a salt tank.
However if you have plans to go with corals, then you'll wanna stick with R/O water. This way, your salt mix and any alk
/Calcium supps are making your water chemistry. Weird calcium, alk
or magnesium levels in your tap water can cause chemistry problems down the road.
As for test kits: I recommend SeaChem or Salifert brand kits...they are the most accurate you can get without forking over serious cash for Hach or LaMotte brands.
During cycling, your only really concerned with ammo and nitrite...and after the nitrIte spike...you can watch nitrates occasionally. You'll end up doing ammo/nitrite tests daily for a couple weeks.
After cycling, that's when you need to start making sure pH is good and steady. You'll also need an alkalinity test kit (carbonate hardness). If you want to keep corals, you'll need a calcium test kit as well. calcium and alkalinity have a somewhat complex relationship (this thread may help you understand it: http://www.aquariumadvice.com/viewtopic.php?t=35749
)...and with stony corals you need proper calcium and alkalinity levels to keep them healthy and growing.
Currently, because I am correcting low calcium levels, I have to test Alk
every day. Eventually, I should only have to test a couple times a week, prior to dosing alk
Also after cycling, you can cut back to weekly ammo/nitrite/nitrate tests. You'll wanna test phosphates too, because strong lighting and a tiny amount of phosphates will give you cyanobacteria (red slime algae).
Ok, now that I've thoroughly scared you with test kits
You only need to worry about an alkalinity kit and a phosphate kit right now...and even then you don't have to have them until the tank is cycled and you prepare to add fish.