I'll try to answer and suggest. I'm going to make the assumption that you are planning a reef tank, and have a fairly large wallet.
75 gallon... Good size tank. Are you going to have a sump or refugium? If so, get the reef ready version, as the All-Glass 75E (the cheaper one) and some others have tempered glass, and can not be drilled.
75-125 lbs rock should be fine for the tank. I'm assuming that this (plus the live sand) will be the main source for biological filtration.
4-4.5 live sand. Is this inches? Didn't your science teacher teach you that units are important???
Assuming that it is, you're at a good depth for a DSB. Check out
this thread though, as there seems to be a return to the thoughts that a bare bottom tank is better. I, myself, have a 3-4 inch sandbed, and no I did not read the 1000 posts either.
You never said if you were going to do a reef tank, with high light corals. I would go with a 2x 250w MH, plus some VHO (2x 110 actinic 03), and you could have most corals. I would use electronic ballasts for the MH bulbs, as they are more efficent, and Ice caps for the VHO. Don't forget fans in your canopy, as these lights get really hot.
For skimmers, I don't really have a suggestion, as it would depend on whether or not you have a sump. AquaC remora pro's are considered by many for to be the best hang on skimmer. For in sump styles, I've heard good things about lifereef and euroreef, but there may be better one's out there.
For a heater, everyone seems to think ebo-jagers are good. I would probably go with 2x 150w. You may also end up with a chiller and a temperature controler if you get real involved, and have a fat wallet.
Thermometer's are always good. Instead of a hydrometer, see about getting a refractometer. They are a lot more accurate. More expensive though ($40 for the cheap ones online)
I use crystal sea's salt. Instant Ocean is used by a lot of people. Some people think that IO is a source of toxic chemicals in the long run. Other's think that Crystal sea's is bad. My suggestion. Choose whatever's cheaper, and don't switch later (as that seems to be where most people have problems)
Test kits: salifert seems to be the most accurate and widely used. I recommend getting ammonia, nitrate, /nitrites, pH/alkalinity, and calcium to start with. It's always a good idea to have a test kit for anything you are dosing into the tank.
Get a RO/DI filter for your tank.
http://www.airwaterice.com has good prices and service. Email them for good advice.
Things not mentioned:
You'll need some sort of current other than the skimmer. This can be solved by powerheads (maxi-jet's are reliable) or the return from a sump.
There are wave maker's available, which simulate the natural randomness of ocean currents. The SQWD looks like a good, inexpensive product that can be used on the sump return, or used in a closed loop with a external water pump.
If you are doing a reef tank, you'll probably be dosing for calcium and alkalinity at some point. There are premixed chemicals, such as B-Ionic which are good (they come in two parts.) but become expensive, especially on larger tanks. You can also use kalkwasser, calcium reactors, and a number of other items to add calcium and alkalinity to your tank. A good article about all this can be found
here
Hope this helps
