Sounds like the start of a decent reef project...established 75 converting to a reef.
Lighting is a open ended topic. It sort of depends on the type corals you prefer to keep. For your purposes with softies and 'shrooms and such you could probably get by with a minimum of 220 watts of either/or PC's&VHO's(thats just for low intensity)....additional lamps would increase your output. Personally I like the combination I use for now. I'm able to keep a large assortment of LPC/softies/'shrooms/polyps/leathers/...even a squamosa clam & a couple of nice SPS corals. I run 4x65 watts of 10k PC's & 2x110 watts of URI super actinic VHO's...on a 55 gal reef.In my opinion my lighting is moderate at best.
There are multitudes of lighting options and combinations...most preferences are PC's/VHO's/MH's...best bet would be to decide what corals you'd like to keep long term. Then design the lighting up-grade to suit your needs.
Just as an example...if I was going to light a 75 reef for leathers/shrooms/LPS corals...I'd install 2x250 watts of Iwaski 6500k MH's & 2x110 watts of URI actinic VHO's...you could build a real nice reef with that lighting set-up. Plus...your imagination is the limit is deciding on a combination of available lighting options to suit your needs.
Not that you'd need it or anything but since you are converting to a reef....check this link out...
Reefkeeping 101 ...its full of bits of information that is very useful in regard to developing a reef. Hope it helps ...
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Especially check out the section on lighting....lots of tech info that may help you make a decision for your reef.Best of luck....
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