A lot of what you are asking revolves around the fish that you want. For clowns 55 is way more than you need (but doesn't hurt). So I would narrow down your list a little more before you jump into a tank.
Sump is a good idea because with more water volume the more stable the system will be and less stressful on your inhabitants. Also cleans up your display tank with less equipment. But if you are choosing "beginner" fish that are more hardy then just paying close attention to water evaporation, water parameters and regular water changes will be sufficiant to eliminate a sump. Tank doesn't need to be drilled as you can buy overflow boxes, but its nice to have a drilled tank...
For fish only declorinated water is fine. RO/DI water won't hurt, at the very least algea will be reduced. If you find algea problems during spring run off then just get grocery store RO/DI water for those few months.
If you are considering clownfish, an anenome is not required. If you want one do your research, some have very slim survival rates. Better lighting will be required aswell. Personally clowns and anenomes go hand in hand for me
