I would say you need more liverock for a 40 gallon tank. Fish use them to hide, you will put some corals on them, and it is also filtration for your tank. General rule of thumb is a pound per gallon. I have a 32 Gallon tank and 20 pounds of liverock and am going to add 10 pounds here soon. Also, most LFS sell RODI water relatively inexpensive. For the love of all that is holy, USE THAT WATER. Your water quality will be more stable and manageable and the water is not adding any unnecessary elements to the tank.
Cant wait to watch your tank flourish!
We are going to be moving in approximately 3 years and are waiting until then to change our 75 gallon freshwater tank to saltwater simply because of moving stresses that come with fish tanks in general. Our freshwater cichlid tank is mostly self sufficient and the fish are resilient, unlike salwater systems. Good luck moving!