Start using distilled to mix saltwater. Remember that water from water changes needs salt, water for topping off does (to top off use just distilled). Do a few water changes with the distilled water saltwater and it should help get the nitrate down (and most likely phosphate and silicate too).
I am not sure how long you were planning on waiting to get corals, but IMe they are about as hardy as fish. If you stick to the hardier ones at first (zoanthids, mushrooms, etc.) you can start adding them soon too.
What fish are you planning on getting?