Absolutes would be:
1. A reef ready tank
2. A sump
3. A good protein skimmer
4. Good lighting that will keep the corals you want healthy. In most cases, it will be T5's (should have individual reflectors) or Metal Halides.
5. RO/DI unit as the water is one of the most important aspects of having a healthy tank. The 75 gpd units have a better rejection rate than the 100 gpd units. (You would think the higher gpd unit is better, but it is not).
6. Refractometer to test your salinity.
7. Test kits with ph, nitrate, nitrate and ammonia to start. However, if you purchase a master test kit with Calcium, Alkalinity, Magnesium and Phosphate, that is fine as you will need those later.
8. Live rock & live sand.
9. Fans (or a chiller) to cool the tank. Fans placed over the sump and blowing towards your lights will cool the tank. You may need a chiller later depending on how hot your lighting is and what temperature your home AC is set at.
10. A heater.
11. Water movement as the fish need oxygen and the corals will need flow. Some suggestions are tunzes, vortechs or powerheads such as maxijet.
12. GFCI electrical outlets such as those in your kitchen and bathroom. If you don't want to install new ones, you can get a GFCI extension cord from Home Depot that will trip if it gets wet. Water and electricity do NOT mix and your safety is the most important. I do not mean a surge protector, here is one (
http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=18026)
Joyce