Ehhhh I wouldn’t go that far. The Nightmare and Magicite dungeons require maxed out chars, and the best weapons and soul breaks. And you need them for every element which means you need a lot of maxed out chars and gear.
Nightmare Dungeons (1.0, not the JP 2.0) have been powercrept and with the gear the game throws at you, easily doable. They're basically gimmick fights, not gear checks.
Magicite Dungeons, other than certain fights (like Bio Odin) can be cleared with poverty setups or with free gear. I myself I'm a F2P and I'm loaded with great soul breaks that make those easily doable and the Lens Shop allows you to buy soulbreaks, including some extremely busted ones that are relevant years after their release.
The best weapons are artifacts, which use a different currency other than the buyable one and are given out on events.
And other than KH characters, any character can be bought with items given weekly on log-in or biweekly on events. Assuming you don't have them from dungeons or events.