RO will probably have nothing in it if its a quality RO unit. Adding supplemented food for corals is about all you will need to worry about. They make a product from KENT that i like to use called Essential Elements (EE) This will take care of replacing what fish will require, but for the most part, i never need to add it.
Ok I have been looking at this thread since it started because I am doing the exact same thing.. Well almost. I am keeping my FW tank and just starting new with a FOWLR 55g. I did it today and here is what I did...
55g with stand, filter, heater, hoods and lights - $150 off Craigslist
15 lbs live rock @ 6.00 per lbs - $90
55g worth of Instant Ocean salt - $18.97
4 20lbs bags of live sand @ 18.00 ea - $72.00
6lbs of Lace rock @ 2.00 per lbs - $12.00
Total - $343.97
Minus - $100 for getting all the new stuff at a Pet Land Grand Opening... =
Grand total of 243.97!!!!
return those damsels, they're going to be mean little bastards that are going to terrorize your tank(i see humbug and domino, two of the nastiest damsels there are)