I have a 20t reef myself.
I would suggest that you use 20-30lbs of substrate for a shallow sand bed.
Approx 20-25 lbs of live rock seems to be a good amount still leaving room for fish.
I am running 2 hydor koralia 240 powerheads and would suggest to use 3 instead
For soft corals, the least amount of light I would suggest is a 2 bulb t5ho light.
A refugium would be a good addition since this is probably one of your first reef tanks.