There is a way to limit big huge powerful spells without a mana bar. Just make the cooldown timer last for 24 in-game hours. For something like "kill everyone in the world spell", maybe a special item could be required, although in general I'm not a fan of spell components for everything. The reason why is because that changes the limited resource from a generic mana you can use for anything to ten zillion specific types of mana that you can only use for this exact spell.
Also, I don't think money should have weight, it's just an annoyance that is not worth the time and trouble it would take to make it not annoying.
