Knowledge, stable tank parameters and money, you also need to be very patient. SPS grows slow, it can take two years to get large colonies, if watching corn grow is exciting, then this is for you. I have a 120 with about 30 small colonies and large frags all fully encrusted on rocks, I am just sitting back and waiting for its to really grow by winter. I check my levels frequently especially the alk. I dropped two-part and went to CA reactor and have everything dead stable.
Always expect costs, I am changing bulbs again and they are only 6 months old and recently noted that my temp probe for the Neptune Apex needs replacement and need a larger heater element to keep temps stable at night when I turn house temp down. There will always be minor stuff that must be done for health of tank and corals. Also just ordered reactor parts for a deluxe dual chamber sulfur nitrate reactor that I will be building later this week, to get nitrates at zero. I had it with bio-pellets had them on two tanks and just dropped them, they took out to much, now I have more nutrients and will get my 5 to 10ppm nitrates to zero on a permanent basis.