Been in this hobby for 14 years so I have made my fair share of mistakes, but if I were starting over again here is what i would tell myself I need :
--> Invest in Quality RODI unit, keeping stable tank starts with the water you put in, and being able to make water when something goes wrong (it will) is MUCH better than running around town trying to find a place open that sells RODI
--> BUY A QUALITY SKIMMER.... I'm not here to argue with anyone as you CAN run a successful tank without one, but the problem is WHY would you? Skimmers are invaluable methods for removing organics from your water BEFORE they break down into nitrate and nitrite, and also it helps oxygenate your water. I have run systems successfully with and without, and I promise you that buying a quality skimmer makes it MUCH EASIER. Again not here arguing with anyone who will post and say that they don't run one and have a great tank etc, Im here as someone who has done both and can assure you skimmers help keep the water cleaner, and help keep dissolved oxygen levels higher. I will never again run a tank without one (haven't for years now) as the benefits to owning a skimmer far outweigh the cost of buying one. Once you see what it pulls out of your water, there is no point to not own one. Most of the people who don't run a skimmer run a plethora of other means of filtration (carbon, GFO, algae scrubbers, etc).... to me a high quality skimmer rated for your tank (and your stocking list) will only help you succeed in this hobby.
--> Buy a quality ATO.... people often fail to realize how quickly salinity levels change with water evaporation. ATO allow your tank parameters to remain stable and solid throughout the week, because again coming from someone who ran without one for years you WILL FORGET TO TOP OFF THE WATER from time to time when life gets busy, and that will result in changes to your tanks salinity. This will add stress to corals and other inhabitants that is not needed. In case you are new and don't know, the salt doesn't evaporate with the water, thus as water evaporates from your tank this causes a higher amount of salt left in the remaining water volume. With an ATO, your tank gets fresh RODI water added as the water level begins to drop, thereby negating any changes in salinity.
--> Buy vortech powerhead with battery backup.... now you can find other quality powerhead type pumps and rig up a battery yourself, but with the vortechs you can simply buy the battery backup and be done there. This keeps flow in the tank when you inevitably have the random power outages from storms etc and will save you from worrying about the tank as much
--> If you want to run carbon (does help keep "fishy smell" out of the water) purchase low dust types (such as ROX) as carbon dust has been linked to HLLE in tangs.
--> DO WATER CHANGES.... I realize this is against what you wanted, but you can live without all the other fancy gear in this hobby if you take the time every two weeks to change simply 20 gallons of water from your tank. I personally do water changes every sunday night, and in all honesty it has become a time of enjoyment and relaxation as its my time to observe tank extremely close, clean out skimmer collection cup, clean the sand bed, and get some fresh saltwater into the system while removing water that is depleted of various trace elements etc. You will read tanks from all over of people who do and do not change their water regularly, but I guarantee you that if you start looking at those with tanks that you admire, most do water changes as it is the single easiest way to keep high quality water and ensure that you don't get a buildup of random heavy metals, or depletion of trace elements and minerals.
Ultimately you don't need the vortech (but its extremely convent with battery backup unit) but everything else I listed above would be the BARE MINIMUM i would run a tank with now days. You can get buy without the fancy lights, without controllers, with cheap powerbeads etc.... but If you want long term success in this hobby without fooling around with algae scrubbers, bacteria culture dosing, carbon, GFO, etc then I recommend quality RODI, SKIMMER, ATO, and SCHEDULED WATER CHANGES REGULARLY.