Do you want a tank with a lot of live rock or more open space? For biological filtration 15 pounds should be enough to keep up with a moderately stocked tank. If you want a simplistic look you can use rock, ceramic rings in a HOB filter or a sump.
No you do not need live sand. Your dry sand will turn into live sand. I used PFS with no issues for a deep sand bed.
I don't think RO/DI is necessary but it is one of those topics that is a never ending debate. Most pet stores will sell RO/DI for 1 a gallon, you might want to go that route.
A sump is a nice thing to have but it is not necessary. All my sumps turned into second tanks to house fish that wouldn't be good in my main tank. I stopped using sumps on my system and enjoy them so much more. Everything is so much cleaner and easier. Less evaporation and salt creep. I cut done on so much extra plugs such as a return pump, fuge light, powerheads and extra heaters. Sumps do have many advantages such as extra water volume, you can put a fuge in their and shake your macro algae in your tank to feed your fish pods and you can hide you equipment. If you have a 40 gallon tank to actually make a difference you will need a 25 gallon sump.
There are tons of ways to set up a tank. It is really up to you how complected you would want to make it and what you want out of the tank.
Here is my 40 breeder ran with HOB filters.Very minimal rock work. The picture is when I had a sump. The video was taken when I went sumpless. I like going sump less. There was no noticeable changes in my nitrate or the why my fish and coral appeared.
