I guess 6 foot tank would need a sump- right away
You would need return pump and heater(s) right away, also suggest at least 2 powerheads.
Thermometer and some water testing equipment, including refractometer.
RODI unit unless you want to brake a bank and your back buying water from LFS. Some containers for mixing salt, rock, either dry or live. You can buy lights and other Stuff later. There is also myriad of small things, bits and pieces for example for plumbing, you would need too.
Skimmer would be next, but not immediately.
Controllers, dosers, ATO, and so on may come later.
All the best.