Hi Dave!
Welcome to SimplyDiscus! Definitely keep reading the forum and asking questions.Best way to get started. A recent discussion we have had..
http://forum.simplydiscus.com/showth...s-in-my-Plan-)
Addresses some of your questions but to recap.. You should be fine with Discus in your tap water. Especially if they are domestics. Your plan on the 75 gal is fine too but be sure to get a good enough group size. ..its not so.much that discus like to be jammed together..its that they do best in groups. Its makes them feel safer and can promote active feeding and good growth. I routinely grow out small discus in 75 g and 125 gal tanks.
Al