Hi, and welcome. As far as how many fish, general rule is 1 inch of fish per 5 gal. so if your fish is 10 inch , then only about 6 or 7 fish.but also remember, that once you get rock in there you really dont have 75 gal of water. ( i may be off a little too) how long is your tank 4 ft.?? as far as lights, it all depends on what type of corals you want. I have a 4 ft tank, and have a 265 wtt power compact fixture and can keep all soft corals. as far as cost, search the net, there are several places that sell them at differant prices.