you definitely have to clean them in salt water. i would have new salt water made up anyway, for the DT. you are going to want a tub or drum or something to have R.O. water stored in and also to mix new salt water. you can't mix it in the tank with fish in it.
as for lighting, do you want to keep corals/anemones/clams, or is this just going to be fish? if it's just fish, you can use pretty much any lighting that will span the entire tank. fish only need light to see their food.
if you want to turn this into a reef tank, then you have a few choices:
1. metal halide lighting.
2. T5 high output lighting
the best and most expensive being LED
. all of the above choices will allow you to keep anything you want under them.
for a 6', 125, i would go with 3 x 250 watt metal halides, or at least a 12 x 39 watt T-5 HO
fixture. you can use two 3' fixtures if need be.
lighting is new and expensive, but uses the least amount of electricity, and doesn't put any heat in the tank. there are many fixtures out there and few of them are worthwhile.