For the wet-dry--you don't need to get one. If you plan on having live rock, and use 3-6 inches of sand as a substrate, the wet-dry will just be more work than you need. The live rock and sand will provide all the bacteria bed that you need.
Tube heaters are fine, you just need to get the right size. The normal recommendation is 3-5 watts per gallon, so a 75 will need somewhere around 350 watts. You could go with 2 200 watt heaters--this will prevent either heater from running too much, and prevent a larger heater from cooking the tank if it sticks on.
As implied above, sand is a preferable substrate to crushed coral. The cc does not provide the anaerobic conditions needed to process nitrates (well, not unless you really want to have 18+ inches of the stuff), and can contribute to high nitrates by trapping waste particles. Any kind of sand will work--you want the smallest grain size you can find, and some types are made of aragonite, which will help buffer the tank. Old Kastle and Southdown are 2 brands to look for at your local home building supply store.
For fish only, normal output fluorescent lights are adequate--they are just for your viewing. Corals vary in their light needs, anywhere from a minimum of 3 watts per gallon all the way up to 10+. However--the quality of light is more important than the quantity--200 watts of normal output is not the same as 200 watts from a metal hallide. Look at power compacts and metal hallide. VHO are a bit cheaper, but don't provide the same quality of light and will limit you some. Non-coral inverts, like snails, shrimps and crabs, don't really care about the lighting much.
