Everyone will have a slightly different set of 'requirements' ..... mine, that have served me well for over a decade are .....
1. 20 to 30Long tank
2. Heater
3, Powerhead
4. Some form of basic lighting
5. ATO
6. HoB filter with pre-seeded media
7. Prime and an ammonia test-kit
8. Assorted PVC fittings for hiding spots
Most of these things are pretty basic. I like to have an ATO as a way to both ensure medications are maintained at the proper dosage and that I can adjust salinity up more easily. I maintain a large canister of seeded biomedia on my main tank so I always have adequate amounts to setup a cycled QT, whether for a planned or unplanned fish purchase.