The light will be fine for fish only.Probly won't seem quite as bright because the salt filters light.The K of your fixture is 7000,most marine tanks run 10,000-20,000K,but again for fish only your light should be ok.
You sure won't need a big RO system for your tank so get a good one if you do invest.Have the DI is important to end result of water quality.
I always plug BRS for RO systems.I got mine from them and am very pleased.Maybe not the cheapest ,but also came with ALL the bells and whistles you will eventually blow more money on,along with 3 different options to hook it up(I didn't need to go get anything to start this up besides a storage container).My system came with 2 HM digital meters to measure volume ,TDS and everything else and float switch to automatically controll the unit.