In reality, if a freind said "I wanna start a SW tank or reef, what are the basic things that I need to get started?" I know what the the OP said, but someone new wouldnt know about LR and lights
Tank of course
In order of importance
1) Powerheads
2) Lighting (for reef, good lighting is a must, for fowlr not so much)
3) Some LR
4) Filtration (ideally a skimmer, but some get away with HOB PF and Canisters)
5) Patience
After these basic needs in an aquaria, then more can be added, ie; sump, return pumps, reactors, fuges, etc.