I'm just now delving into LEDs myself, thinking of doing an LED system for my 20g nano. This is just stuff from what I have read, but it's not going to be as detailed as some of the other members can get into, for sure!
My tank is 24" long, so I'm building a fixture with a 23" heat sink. Your 75g should be 4ft long, so you would use two heat sinks, 23" long each. My small system is going to require 3 drivers. Drivers are the "power controllers" for the LEDs to run (basically, ballasts) I think I'm going to go with 3 ELN drivers, but you might do better with to HLG drivers. Other people can tell you better why I think that.
For my little setup, I'm considering all different types of color options. There's cool whites, warm whites, blues, royal blues, ultraviolate, reds and greens. By having a setup that only uses cool whites and royal blues, some people have said that it "washes out" certain colors. That's the reason I am going to go with a few different color options. I'm thinking of going with twelve 3w cool whites on one driver; twelve 3w royal blues on another driver, and then in the center I'm thinking about doing three reds without optics, three greens without optics, two ultraviolates without optics. Then, that gives me enough space on the heat sink for four moonlights. I'm by no means saying that it's the only way to do it, but the way I am considering for my LED setup to consist of.
A lot of the pre-drilled heat sinks, LED stars, drivers and so forth, can be bought though one particular company that is standing out among the rest, but you may find it cheaper to go with several different companies for various things. Don't forget that you may want a couple dimmer switches to get your desired kelvin level right.
I hope this bump helps you.