40 gallon reef startup ideas

[JoeMan]

Hey, Im thinking of building a 24"x24"x 16" square reef tank, which comes out to about 40 gallons. Ive been keeping cichlids for a while but wanted to branch out into saltwater, and in particular having a reef tank.

My questions are:

What species of fish would do best in a 40 reef

What kind of filtration is best for this tank setup

My tap water is hard as heck. Can I use this or would I need an RODI system?

Thanks!